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