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 
FYI: I don't think this will work with Split courses.  Split course ids have `:` and `+` characters in them.  And here, we are creating illegal file names with `: and +` since they will not be replaced with `_`.
@dmitchell Do you agree?