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
How about "Note that student.course_groups.filter() creates a fresh query, therefore negating the performance gain from prefetch_related()"?