- retires toggle DISABLE_DEPRECATED_LOGIN_POST
- permanently removes /login_post

Now that studio signin has been retired, we are able to remove the
unused /login_post endpoint.

ARCH-1253

This completes the work started in https://github.com/edx/edx-platform/pull/19453
to use the LMS login and registration for Studio, rather than Studio
providing its own implementation.

LMS login/registration are being used for the following reasons:
1. LMS logistration properly handles all SSO integrations.
2. A single logistration is simpler to maintain and understand.
3. Allows Studio to work more like all other IDAs that use LMS
logistration.

The original switch to use LMS logistration for Studio also added the
toggle `DISABLE_STUDIO_SSO_OVER_LMS` to provide the community some
additional time for switching. This commit removes this toggle, which
at this point means all deployments will use the LMS logistration.

This change requires sharing cookies across LMS and Studio. Should that
prove to be a problem for certain Open edX instances, there are
discussions of possible alternative solutions.
See https://github.com/edx/edx-platform/pull/19845#issuecomment-559154256

Detailed changes:
* Fix some Studio links that still went to old Studio signin and signup.
* Remove DISABLE_STUDIO_SSO_OVER_LMS feature toggle.
* Remove old studio signin and signup pages and templates.
* Fix url name "login", which had different meanings for Studio and LMS.
* Use the following settings: LOGIN_URL, FRONTEND_LOGIN_URL,
FRONTEND_LOGOUT_URL, and FRONTEND_REGISTER_URL.
* Redirect /signin and /signup to the LMS logistration.
* Add custom metric `uses_pattern_library`.
* Add custom metric `student_activate_account`.
* Add Django Settings to allow /signin, /signup, and /login_post to be
disabled once ready.

This work also relates to ARCH-218 and DEPR-6.

ARCH-1253

PROD-1026

Required these be added to CMS since enterprise is installed in the CMS
and there are import failures when these keys don't exist

Improves navigation within Studio for Learning Sequences, speeding up authors who want to see how a learner progresses through content without needing to jump over to the LMS.

This adds a dropdown section navigator to the breadcrumbs on the unit page and copies the sequence navigator from LMS to the studio unit page.

For TNL-6938

Fixing decoding issue.
Fixing flaky unit test.

DISCO-1422

The access settings modal should not have an editable title. Also, the title of the modal was not being interpolated correctly.

* fix type mismatch in third_party_auth migrations

* fix type mismatch in verify_student  migrations

* fix type mismatch in video_config  migrations

* fix type mismatch in verified_track_content  migrations

* fix type mismatch in commercemigrations

* fix type mismatch in xblock_config migrations

* fix type mismatch in course_creators migrations

* fix type mismatch in contentstore migrations

hashlib produces different md5 hash in python3 against same values as
compared to python2.

Revert "Fix code quality test failures"

This reverts commit 8c55e11d.

Revert "Fix celery send_activation_email task failure"

This reverts commit 810eea0e.

Revert "Convert Account Activation Emails to edx-ACE"

This reverts commit 7984c37a.

We need to do this because when I tride to go to the JSON serializer a
bunch of tests started failing because various parts of our code are
putting things into the session that are not JSON serializable.

We can't keep using the default pickle serializer because it defaluts to
using the highest available protocol and that will cause issues with the
python 2 to 3 upgrade since both will be running in production at the
same time.  We need to use a version of the pickle protocol that both
can use interchangably.

We also need to make sure we read with latin1 encoding to make datetimes
work correctly between the two versions of python.

This will force a logout as sessions fail to load but this should be a
more performant and secure serializer moving forward.  The reason we
overwrote it in our config is that it previously used to be the default
and we didn't want things to breake and force logouts when we changed
it.  We're no more okay with people getting logged out.

Fixing byte issues.

The API documentation decorators do not have to leak which solution we
use to generate the docs. Here, and as discussed in PR #21820, we rename
the `openapi` module to `apidocs`, and we make sure that this module
includes all the right functions to document API Views without referring
to Open API.

Add publisher link to settings dropdown if the features setting
is set.

The _id comes back as a dict and it's in the wrong order compared to the
dict stored in mongo.  This results in lookups for deletion failing when
we use the '_id' as the lookup key.

Luckily the delete function also takes the asset_key as a valid key to
delete by so we just use that instead.

There is a bug in the underlying management/base code that tries to make
all manageent command output be unicode.  This management command
outputs the binary tar file data and so breaks in python3.  In python2
the code is happy to pass bytes back and forth and in later versions of
django this is fixed.  Howevere it's not possible to get this test to
pass in Python3 and django 1.11