Move content_libraries to cms and mark as stable

[kdmccormick]

Background

The content_libraries django app, exists in the openedx/ source dir, which is used by both LMS and CMS. This means that all content_libraries-related code is available to be imported and invoked in LMS, which could lead to some unintended bugs or architectural issues down the line.

Opinion: Content libraries are used by authors, but NOT learners or instructions. The entire purpose of content libraries is for authors to create content that is NOT visible to the LMS, and then use that content in their courses. There is no reason that content-libraries-supporting code should need to execute in LMS. (We don't have consensus on this opinion.)

Tasks

Figure out whether content_libraries belongs in openedx/ or cms/.

If it belongs in cms/:

• Move https://github.com/openedx/edx-platform/tree/master/openedx/core/djangoapps/content_libraries under cms/djangoapps. This will require some refactoring, as there are probably already pieces of LMS that use pieces of the content_libraries app.

Details

TBD. Needs refinement.

[kdmccormick]

(and mark some of it as stable)