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

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.

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.

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, 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.