[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 
The get_cohort function really isn't buying us much here. Given the uneasiness that I feel about having a default-valued parameter controlling whether the function has a side effect, I think it would be cleaner to just use the model directly here.