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 
I don't know anything about the `ReportStore` nor its existing uses, but is there a reason it can't use the correct serialization? (slashes, colons, plusses, at signs)