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
if i understand correctly, it's already the case that the entire course roster is fetched into memory (and cached) when generating the csv.  adding the prefetched relation means we will pull a second similarly-sized result set onto the server, and join the two in memory.  this shouldn't go to production if we aren't confident that there's a sane execution plan.