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 
But anyway, for the specific purpose here, I would just do something like what is inside the try -- you shouldn't have an issue with malformed course_id strings.