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 
Since you are already parameterizing the other fields, why not also parameterize the `kwargs`?  Instead of adding a conditional check for one ddt value?