Assign cohorts to groups in the instructor dashboard

TNL-653
I dug around and couldn't find any consistency. The API documentation suggests that we should consider making this a paginated interface, but doesn't say what that looks like:

https://openedx.atlassian.net/wiki/display/AC/API+Thoughts

I think you should probably leave it as is for now, and in the future we should start to use the Django REST Framework for all of our APIs. For an example, see how DedX is using it in the user_api:

https://github.com/edx/edx-platform/blob/master/openedx/core/djangoapps/user_api/views.py#L168