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

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.

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

This implements the "following" value for the "view" query parameter on
the thread list endpoint.

This will allow clients to determine what actions (e.g. following,
endorsing, and editing content) a user can take on a piece of content.

The thread list endpoint now accepts the text_search parameter and also
includes a text_search_rewrite field in its responses.

This endpoint returns course metadata relating to discussions as a
starting point for clients.

This is done via PATCH on a comment instance endpoint.

This is done via PATCH on a thread instance endpoint.

Also added thread_list_url field.

This allows authors to follow the thread immediately upon creation.

Also, fix the field-specific error format for all API endpoints.

This requires cs_comments_service commit fdf017c918.

The checks are now within the Python API instead of the DRF view. This
will be necessary for certain operations (like fetching/editing threads)
because the relevant course cannot be known until the thread is fetched
from the comments service. This commit updates the existing endpoints to
fit that pattern.

This is an initial implementation that only allows retrieval of all
threads for a course and only returns an easily computed subset of the
fields that are needed, in order to keep this change from getting too
large.

JIRA: MA-641

This is the first stage of the new Discussion API.

JIRA: MA-604