Fix bug: re-render is triggered too many times in textbooks.

Bump edx-enterprise version

Message related to certificate on courseware progress page are translated to platform language only. Part 2 of 2.

page related to certification and course mode. Part 2 of 2.

Use ugettext_lazy instead of ugettext to ensure the strings shown are in
the language the user chose. Otherwise the strings are in the platform
language.
Keep the original ugettext for function financial_assistance_form.
The test for that function fails if the text is not in english.

* INCR-413: Run python-modernize and isort

* INCR-413: Fix for text_type usage

bump edx-enterprise to version 1.6.8

Added Staff Graded Points XBlock

Revert "Use drf-yasg for Open API documentation"

Remove the course name from the enroll button.

Replace the course name and change the button text to ‘Enroll Now’. The course name appears above this button.

Use case: The Threat of Nuclear Terrorism course team would prefer "Enroll Now” instead of "Enroll in Nuclear Terrorism" because of various interpretations of what "enrolling in nuclear terrorism" could mean.

Use drf-yasg for Open API documentation

* INCR-250: Make compatible with Python 3.x without breaking Python 2.7 support --> openedx/core/djangoapps/programs

* INCR-250: Disable pylint warning and replace _f placeholder with actual name

* INCR-250: pylint format correction and fix over length line limit

Mergeback PR from private to public.

Bumped edx-enterprise version

Python Requirements Update

The rate limiter counts requests in a 5-minute window.  To be sure we
aren't hitting edge cases, make the future requests 6 minutes plus 1
second in the future.

The error in the test was:
```
AssertionError: The `request` argument must be an instance of `django.http.HttpRequest`, not `rest_framework.request.Request`.
```
The (controversial) incompatible change was in 3.7.4: https://github.com/encode/django-rest-framework/pull/5618

I'll look into whether there's another way to address it.

<details>
  <summary>Full error report</summary>

```
AssertionError: The `request` argument must be an instance of `django.http.HttpRequest`, not `rest_framework.request.Request`.

Stacktrace

self = <lms.djangoapps.discussion_api.tests.test_views.CommentViewSetListTest testMethod=test_profile_image_request_for_null_endorsed_by>

    def test_profile_image_request_for_null_endorsed_by(self):
        """
        Tests if 'endorsed' is True but 'endorsed_by' is null, the api does not crash.
        This is the case for some old/stale data in prod/stage environments.
        """
        self.register_get_user_response(self.user)
        thread = self.make_minimal_cs_thread({
            "thread_type": "question",
            "endorsed_responses": [make_minimal_cs_comment({
                "id": "endorsed_comment",
                "user_id": self.user.id,
                "username": self.user.username,
                "endorsed": True,
            })],
            "non_endorsed_resp_total": 0,
        })
        self.register_get_thread_response(thread)
        self.create_profile_image(self.user, get_profile_image_storage())

        response = self.client.get(self.url, {
            "thread_id": thread["id"],
            "endorsed": True,
>           "requested_fields": "profile_image",
        })

lms/djangoapps/discussion_api/tests/test_views.py:1446:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/test.py:291: in get
    response = super(APIClient, self).get(path, data=data, **extra)
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/test.py:208: in get
    return self.generic('GET', path, **r)
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/test.py:237: in generic
    method, path, data, content_type, secure, **extra)
../venvs/edxapp/local/lib/python2.7/site-packages/django/test/client.py:416: in generic
    return self.request(**r)
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/test.py:288: in request
    return super(APIClient, self).request(**kwargs)
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/test.py:240: in request
    request = super(APIRequestFactory, self).request(**kwargs)
../venvs/edxapp/local/lib/python2.7/site-packages/django/test/client.py:501: in request
    six.reraise(*exc_info)
../venvs/edxapp/local/lib/python2.7/site-packages/django/core/handlers/exception.py:41: in inner
    response = get_response(request)
../venvs/edxapp/local/lib/python2.7/site-packages/django/core/handlers/base.py:249: in _legacy_get_response
    response = self._get_response(request)
../venvs/edxapp/local/lib/python2.7/site-packages/django/core/handlers/base.py:187: in _get_response
    response = self.process_exception_by_middleware(e, request)
../venvs/edxapp/local/lib/python2.7/site-packages/django/core/handlers/base.py:185: in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
../venvs/edxapp/local/lib/python2.7/site-packages/django/utils/decorators.py:185: in inner
    return func(*args, **kwargs)
../venvs/edxapp/local/lib/python2.7/site-packages/django/views/decorators/csrf.py:58: in wrapped_view
    return view_func(*args, **kwargs)
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/viewsets.py:95: in view
    return self.dispatch(request, *args, **kwargs)
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/views.py:494: in dispatch
    response = self.handle_exception(exc)
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/views.py:491: in dispatch
    response = handler(request, *args, **kwargs)
lms/djangoapps/discussion_api/views.py:505: in list
    form.cleaned_data["requested_fields"],
lms/djangoapps/discussion_api/api.py:659: in get_comment_list
    results = _serialize_discussion_entities(request, context, responses, requested_fields, DiscussionEntity.comment)
lms/djangoapps/discussion_api/api.py:468: in _serialize_discussion_entities
    request, results, usernames, discussion_entity_type, include_profile_image
lms/djangoapps/discussion_api/api.py:413: in _add_additional_response_fields
    username_profile_dict = _get_user_profile_dict(request, usernames=','.join(usernames))
lms/djangoapps/discussion_api/api.py:350: in _get_user_profile_dict
    user_profile_details = AccountViewSet.as_view({'get': 'list'})(request).data
../venvs/edxapp/local/lib/python2.7/site-packages/django/views/decorators/csrf.py:58: in wrapped_view
    return view_func(*args, **kwargs)
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/viewsets.py:95: in view
    return self.dispatch(request, *args, **kwargs)
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/views.py:477: in dispatch
    request = self.initialize_request(request, *args, **kwargs)
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/viewsets.py:118: in initialize_request
    request = super(ViewSetMixin, self).initialize_request(request, *args, **kwargs)
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/views.py:381: in initialize_request
    parser_context=parser_context
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <rest_framework.request.Request object at 0x7f597c773890>
request = <rest_framework.request.Request object at 0x7f597fa20f90>
parsers = [<openedx.core.lib.api.parsers.MergePatchParser object at 0x7f59810c32d0>]
authenticators = [<edx_rest_framework_extensions.auth.jwt.authentication.JwtAuthentication object at 0x7f597c713690>, <openedx.core.lib...rest_framework_extensions.auth.session.authentication.SessionAuthenticationAllowInactiveUser object at 0x7f597c773390>]
negotiator = <rest_framework.negotiation.DefaultContentNegotiation object at 0x7f597fa20410>
parser_context = {'args': (), 'kwargs': {}, 'view': <openedx.core.djangoapps.user_api.accounts.views.AccountViewSet object at 0x7f597c7846d0>}

    def __init__(self, request, parsers=None, authenticators=None,
                 negotiator=None, parser_context=None):
        assert isinstance(request, HttpRequest), (
            'The `request` argument must be an instance of '
            '`django.http.HttpRequest`, not `{}.{}`.'
>           .format(request.__class__.__module__, request.__class__.__name__)
        )
E       AssertionError: The `request` argument must be an instance of `django.http.HttpRequest`, not `rest_framework.request.Request`.

../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/request.py:159: AssertionError
```
</details>

In production, we use UTC as the time zone.  DRF 3.7.7 now puts all
times in the currently set timezone where it used to use UTC.  By
setting TIME_ZONE to UTC, we keep the same results we used to get.

In a few places, we had to change the expected test results to be UTC.

Because we added drf-yasg and pinned djangorestframework

* Install drf-yasg

* Add drf-yasg settings and urls

* Pin drf to make drf-yasg work

* Adjust config-models version to be compatible

* Remove django-rest-swagger (the old way)