Add a new user API for the notifier

This new endpoint is designed specifically to fill the needs of the
notifier and should not be used by other clients.

@jimabramson @nasthagiri 
For better readability, consider using Model.objects.create instead (which also saves the model).  I believe it would be more suited to the Django-style.  So the code would be consolidated to:
```
moderator_perm = Permission.objects.create(name="see_all_cohorts")
moderator_role = Role.objects.create(name="Moderator", course_id=course.id)
```