Show assigned cohorts on the instructor dashboard

TNL-164

Upcoming in follow-up PR: alerting users that they can get cohort data from the data download section.

@jimabramson @nasthagiri please review.
FYI: @gwprice @andy-armstrong @cahrens 

[Sandbox](http://dan-f.m.sandbox.edx.org/courses/DataDownloadX/CohortsCSV101/2014/instructor#view-data_download) for @explorerleslie
Actually, if we are not careful, we wouldn't just *double* the database queries, which is currently constant, but they would grow linearly with the number of students (no way would that scale).  

I agree with @jimabramson that we should test this locally before it goes to production.  For the mongo database, we had created a contextmanager (`check_mongo_calls`) that we can wrap around unit tests to make sure the number of database queries remain as expected.

If we find that there is a potential scalability issue with this, then we should implement this as a background celery task (like we already do for creating the Grades CSV).