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
That's a good idea. To be honest, though, I'm not entirely positive why the number of queries is 2 in terms of how SQL works. However, based on my testing, it looks like there's 1 query for the `User.objects.filter(...).select_related('profile')`, and an additional query for the ``.prefetch_related('course_groups')`. Is that explanation sufficient?