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 
Actually, I thought I was cleaning up the `course_id.to_deprecated_string()` occurrences. This one seems necessary, right?