REV-1204 Add translation by marking string as translatable w/o translating at runtime

In order to enable us to remove flag_undefined_default, this
updates flags according to alternatives documented in:
https://github.com/edx/edx-platform/blob/master/openedx/core/djangoapps/waffle_utils/docs/decisions/0001-refactor-waffle-flag-default.rst

These flags were updated with a migration:
- enable_checklists_quality
- REJECTED_EXAM_OVERRIDES_GRADE
- ENFORCE_FREEZE_GRADE_AFTER_COURSE_END
- WRITABLE_GRADEBOOK

ARCHBOM-1304

PROD-1565

The temporary waffle flag ENABLE_VIDEO_URL_REWRITE is no
longer needed, becaues the rollout is complete.
See https://openedx.atlassian.net/browse/PROD-62

ARCHBOM-1304

Added `is_enrolled` field to check if user's enrollment is active.

- Have it load dates from edx-when, not just write to it. This
  fixes self-paced courses where edx-when is only place dates are
  kept.
- Have it read original date for a homework from edx-when when
  resetting a date. This fixes the message it gives the instructor
  about whether it was successfully reset.
- Have it recursively set a date, rather than assuming that dates
  are only ever set on the subsection layer. This fixes setting
  dates on self-paced courses (where dates are set all the way
  down) and just in case somebody somewhere edits the course xml
  to have a date where it's not expected.

Updating edx-when version to pull in a change related to not
returning dates if the enrollment happened after course end
(if no enrollment end date is set)

This PR solves the DeprecationWarning mentioned in:
https://build.testeng.edx.org/job/edx-platform-python-pipeline-master/warning_5freport_5fall_2ehtml/

HTMLParser was renamed in html.parser in Python3:
https://docs.python.org/2/library/htmlparser.html#module-HTMLParser

* html_parser.HTMLParser().unescape from six.moves has been deprecated

* instead use html.unescape from Python3

Documentation for unescape in Python3:
https://docs.python.org/3/library/html.html#html.unescape

- html_parser from six.moves has been deprecated

- instead use html.parser from Python3

- Order imports using isort

- Delete unused import crum

Fixing 56 GuessedAtParserWarnings, in commit edx#24098

Background: BeautifulSoup automatically picks the fastest parser available. By default, it picks the "lxml" parser.

Per the [BeautifulSoup](https://www.crummy.com/software/BeautifulSoup/bs4/doc/#installing-a-parser) documentation:

> Beautiful Soup supports the HTML parser included in Python’s standard library, but it also supports a number of third-party Python parsers. One is the lxml parser. Depending on your setup, you might install lxml with one of these commands.
> Another alternative is the pure-Python html5lib parser, which parses HTML the way a web browser does. 

Context: We changed two statements, one in lms and another in openedx. Both statements fire up BeautifulSoup. Now we explicitly ask for "lxml," following the recommendation on BeautifulSoup's documentation:

> If you can, I recommend you install and use lxml for speed. If you’re using a very old version of Python – earlier than 2.7.3 or 3.2.2 – it’s essential that you install lxml or html5lib. Python’s built-in HTML parser is just not very good in those old versions.

Before:
`soup = BeautifulSoup(content)`

After:
`soup = BeautifulSoup(markup=content, features="lxml")`

The warnings are gone, tests are passing in local.

* Hide private team-sets from users not on a team

* Modify add team count to factor in team visibility

* Fix bug that broke search w/in private team-sets

to remove some `DeprecationWarning`s from the logs
as the former method as been deprecated since Python 3.3 [1][2].

- [1] https://github.com/python/cpython/blob/3.3/Lib/logging/__init__.py#L1252-L1253
- [2] https://stackoverflow.com/a/15655674

* use external_user_key in teams csv download

- Add a new CourseEnrollmentCelebration model, which ties a
  course enrollment to some booleans about progress celebrations
- Add serialization of the new model to the existing courseware_api
  app's existing course info view
- Add new API in courseware_api to update a celebration model

MST-234

- add serializer mixin to expose date banner info
- add endpoint to reset deadlines for a sepcific course

- yt_video_metadata returned a generic non-json-api-friendly 500 error
  when called on a non-youtube video
- load_metadata_from_youtube was crashing when called from the xblock
  yt_video_metadata endpoint. It passes a webob request, which has a
  different api for retrieving the http referer.

In an earlier PR, we moved logic from process_view to process
request, so mapping would happen earlier in the middleware
lifecycle, and the code_owner metric would be set for requests
that never made it to process_view.

The temp_view_func_compare custom metric was added temporarily to
ensure this earlier refactor did not introduce any unaccounted for
differences. It did not, so we are removing this temporary metric.

ARCHBOM-1263

Only actually reset user schedule if they have missed deadlines and not missed any gated content deadlines

* conservative move to process_request
* added temp_view_func_compare metric to be extra conservative

ARCHBOM-1263