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
have you confirmed how the db driver is going to execute this, under the hood?  

after reading this https://docs.djangoproject.com/en/1.4/ref/models/querysets/#prefetch-related i think there are caveats - we should ensure we're not stepping on a landmine here.

should take into account whether the original implementation scaled acceptably to full-size courses - i don't know offhand if that is the case.