Task - RBAC AuthZ - Implement automatic policy cleanup when a scope is deleted

[dwong2708]

To maintain data consistency, implement a role cleanup mechanism on course deletion. For example, when a course is deleted, any role associations linked to that course should be automatically removed.

[bmtcril]

FWIW I don't think we have a UI to delete courses, but I'm sure it can be done and admins do it somewhere. I think if we have a CourseScope that FKs to CourseOverview we should just be able to count on the cascade delete to clean up, but we should definitely confirm that.

[rodmgwgu]

This is now covered by #209 (review)