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
FWIW one way to avoid conditional asserts would be to convert `userreports` into a dict
```
cohorts = dict((r['username'], r['cohort']) for r in userreports)
self.assertEqual(cohorts[cohorted_student.username], cohort.name)
self.assertEqual(cohorts[non_cohorted_student.username], '[unassigned]')
```