* Add "reason" JSON-encoded field to CreditRequirementStatus.
* Add a "failed" status to CreditRequirementStatus.status choices.

The CreditRequirement "configuration" field was renamed in Django
code without updating the migration.  This commit adds a second
migration to delete the incorrect field and add a new field.
The credit feature is currently disabled in production, so
the model isn't currently used by application code.

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.

Adding the pyuca to support sorting in differnet languages.

TNL-2047

Also, I replaced a number of tearDown methods with addCleanup instead.
And also remove some unneeded patch.stopall() calls.

* Handle grading errors

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

With the old logic, we weren't running pylint on openedx at all.  We
were also running it on lms/lib/comment_client twice, and on the
installed xblock third-party library.

This is the first stage of the new Discussion API.

JIRA: MA-604

TNL-2073

TNL-1851