chore: Add django admin for RestrictedCourseMetadata

[pwnage101]

ENT-9569

Also, this should be a usability/QoL improvement for catalog, catalog query, and content metadata fields in other admin pages.

Screenshots

List RestrictedCourseMetadata:

• Note: first column links to record, 2nd and 3rd columns link to other objects.

Edit a restricted course (RestrictedCourseMetadata):

Edit a restricted run (ContentMetadata):

Edit a course with lots of catalog/catalogquery relationships (ContentMetadata):

• Note that "Catalog Queries" and "Enterprise Catalogs" are clean lists now (instead of multiple objects on one line), and link back to the object.

[pwnage101]

If I had more time tonight, I'd make a course in ContentMetadata list out all its restricted versions.