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
