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.

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

Add more extensive testing

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 comments service rejects whitespace-only content for certain
strings, so the discussion API should also reject such content.

Some old threads do not have the pinned field set, which the thread
serializer should handle.

The rendering in this commit does not prevent MathJax content from being
rendered as the discussion JavaScript code does.

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 bug caused any attempt to update a comment with a non-null
parent_id to result in a 500 error without applying the update. The bug
was introduced in commit 2c7590d1.

This is done via PATCH on a comment instance endpoint.

This depends on cs_comments_service commit 0487891.

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.

This will make it easier to add the creation and update interfaces.

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.

The API's type field should come from the CS data's thread_type field,
not its type field, which distinguishes comments from threads.

This commit adds fields that are related to the requesting user's
interaction with the thread (e.g. following).

The thread list endpoint needs to restrict the set of threads it
retrieves if the course is cohorted and the user is not privileged.
This also adds an explicit default sort (by most recent activity).

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