Add the ability for Microsites to be able to send appropriately branded ...

...emails rather than the default system installed version

fix tests. Update Admin views/forms.

enforce uniqueness constraint in the Admin ModelForm submissions
Note to reviewers, seems like the change to OpaqueKeys is causing an update on a few course_id fields in the SouthMigrations. While this is not needed for this feature contribution, it seems like - since this is the first migration post-OpaqueKeys - these are getting picked up in this migration.