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 
As a follow on to the `AlreadyRunning` question:

If a student-profile download task for a course is already in progress, but then the celery worker dies (or some reason the task never completes), doesn't the task always remain as `AlreadyRunning` and can no longer be restarted?  Or is there a way in the UI to delete a previously incompleted task?