Added missing unit tests.

* Upgrade edx-submissions
* Upgrade edx-ora2
* Upgrade edx-val
* Upgrade edx-proctoring
* Update all edx-platform code that depends on DRF, including:
  - auth_exchange
  - cors_csrf
  - embargo
  - enrollment
  - util
  - commerce
  - course_structure
  - discussion_api
  - mobile_api
  - notifier_api
  - teams
  - credit
  - profile_images
  - user_api
  - lib/api (OAuth2 and pagination)

* Upgrade edx-submissions
* Upgrade edx-ora2
* Upgrade edx-val
* Upgrade edx-proctoring
* Update all edx-platform code that depends on DRF, including:
  - auth_exchange
  - cors_csrf
  - embargo
  - enrollment
  - util
  - commerce
  - course_structure
  - discussion_api
  - mobile_api
  - notifier_api
  - teams
  - credit
  - profile_images
  - user_api
  - lib/api (OAuth2 and pagination)

refactor community_ta and staff in docstrings

MA-1212 - added response count field in tests

removed unneccessary function

"MA-1212 - added response count in thread retrieval service and updated tests"

updated serializerz test for response count

MA-1212

exclude response count from thread get list

removed response_count field from update and create thread services

made resp_total dynamic for cs thread in test_serielizer

quality fixed

improved quality

add TODO JIRA ticket on removed fields

An incorrect response was being returned when voting for a comment
or thread.

MA-1182 - added test for not enrolled in course, removed commented out nose test code, formatted import statements according to edx standards

These signals are caught by the Teams app and used to update
`last_activity_at` on both teams and individual users.

TNL-2497

Currently when GETting comments, the thread of the comment will be
marked as read. This change makes this effect optional as well as
setting the default to not mark the thread as read.

Discussion API for thread sorting now takes the sort_by
parameter for the sort key such as by comment_count or
vote_count. Also takes the sort_direction parameter for
sorting the thread results.

This is a new TestCase base class, intended for situations where
you only want to initialize course content in the setUpClass(),
as opposed to setUp(). This is done for performance reasons, since
many test cases only use courses in a read-only fashion,
particularly in LMS tests.

This commit also converts the following modules to use this new
base class:

  lms/djangoapps/ccx/tests/test_ccx_modulestore.py (38s -> 4s)
  lms/djangoapps/discussion_api/tests/test_api.py (2m45s -> 51s)
  lms/djangoapps/teams/tests/test_views.py (1m17s -> 33s)

Discusion API now takes a view parameter when getting a thread list.
This view parameter only takes "unread" or "unanswered" as possible
values.

This also fixes two bugs:
* A thread created by a non-privileged user in a cohorted course would
  not have group_id set
* Any user could set the endorsed field on creation of a comment

Flagging/unflagging is done by issuing a PATCH request on a thread or
comment endpoint with the "abuse_flagged" field set.