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 
Optional: It would be "nice" if this common code between the grades report and the student profile report were shared.  For example, you can create a common helper that takes in (1) the function to call and (2) the substring for the success and error messages.