Assign cohorts to groups in the instructor dashboard

TNL-653
Makes sense. It is still odd that we are accepting posts, puts and patches and treating them all the same way. We should get consistent about this, which we will be forced to do if we use the Django REST Framework since you implement the POST methods directly. For example:

https://github.com/edx/edx-platform/blob/master/openedx/core/djangoapps/user_api/views.py#L253