Add CourseApp for Learning Assistant feature and register entrypoints in LMS and CMS..

This commit adds a concrete implementation of the CourseApp ABC from the CourseApps edx-platform Django app. This enables the Learning Assistant to be represented by a card on the Studio Pages & Resources pages. Please see the associated ADR for more details.

This commit also registers this CourseApp class as an entrypoint to the CourseApps edx-platfrom Django app. Because the CourseApps REST API is a part of the CMS, the Learning Assistant plugin was also added as entrypoint to the CMS.