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 can see that be useful functionality to include as a utility in the opaque keys library. Although, generating a safe filename from arbitrary input is really a more general question. Also, should the filename be a reversable mapping? Or just readable? If you want the former, @pmitros wrote a version for xblock: https://github.com/edx/XBlock/commit/2577b70591cc73582ac059e29161dc37efe60f2c#diff-cef7a9b520121ab3e6f4e83a4ca297c7R942