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.

The string "</script>" in JSON data would end the script element we're
embedding the data in.  To make sure the data doesn't disrupt the
script, we escape the slash as \/ .

Removed inappropriate section tags. Because screen readers announce each section, they should not be used unless they are actually defining meaningful sections.

[LMS-57]