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 
Yes, I agree. However, I chose to follow the existing convention (`push_grades_to_s3`), as we seem to be putting off the other rename-to-generalize refactors until after this story lands. At the same time, it probably wouldn't be too much trouble to rename both of these functions. I'd be happy to go either way.