Background-ify, include cohort column in student profile CSV download

TNL-453

Re-implement the existing student profile info CSV download (on the "Data Download" section of the instructor dashboard) as a background task to alleviate browser timeouts. Also add a "cohort" column to the CSV.

@nasthagiri 
Yeah, I think consolidating it in that method makes sense for now.  If in the future, we want to have different conventions, we can parameterize it then.