Adds a read-only mode to the discussion module which disables any
controls for updating the discussion (votes, follows, replies,
etc). This mode is enabled for users who are not a member of the team,
and are also not moderators, admins, or community TAs.

TNL-2487

Authors:
  - Daniel Friedman
  - Ben McMorran
  - Peter Fogg

TNL-1892

TNL-2305

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

IsAuthenticatedOrDebug hides potential issues with API client code that is run in local environments and later deployed to production where authentication fails.

XCOM-193

TNL-2321

This is done via PATCH on a thread instance endpoint.

TNL-2248

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

This requires cs_comments_service commit fdf017c918.

This allows course authors to choose between two difference licenses for their
course content: All Rights Reserved, or Creative Commons. In the backend, XBlocks
that wish to allow custom licenses need only inherit from LicenseMixin, which
adds a `license` field as a string.

License information is displayed in the Studio editor view, and just below the
rendered XBlock in the LMS. In addition, if the course block itself has a custom
license set, this license will display just below the main body of the page
on courseware pages.

This entire feature is gated behind the LICENSING feature flag.

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

TNL-1537

Co-Authored-By: Andy Armstrong <andya@edx.org>
Co-Authored-By: cahrens <christina@edx.org>

This makes these files importable, and puts them into the new best guess
as to where files should live.

TNL-1493

See https://openedx.atlassian.net/wiki/display/TNL/User+API for details

XCOM-107