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 
There was discussion earlier about using "generated" instead of "calculated" in the text around the button.  Does it make sense to use that here as well?  And if so, you could also change the method name to "generate_..." to be more broadly consistent.