Add history table & activate/deactivate admin actions for OrganizationCourse
We needed an interface to bulk activate/deactivate organization-course linkages. This version implements one in the Django Admin panel, based on the existing admin action for activating/deactivating organizations.
Also, we add history tracking to the OrganizationCourse model. Unfortunately, changes made via bulk edit will not be reflected in the history table due to limitations in Django ORM's bulk-write API.
For screenshots, see #169