[WIP] Show assigned cohorts when viewing all students on the "data downloads" section of the instructor dashboard

TNL-164

@jimabramson @gwprice 

FYI: @cahrens @andy-armstrong @nasthagiri 
@nasthagiri @gwprice Is this the proper way to pre-fetch the related course_groups for each student in students? My only concern is if chaining the call to `prefetch_related` after `select_related('profile')` is a problem.