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 
Good point. I'm not sure whether this falls into the TNL-525 "Make instructor_task upload terminology more generic" task. @nasthagiri ?