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 
Since "Cohort" has many definitions and may not be obvious to translators, you should create a "Translators" comment above this line and it will get picked up Transifex.  For example:
`Translators: 'Cohort' refers to`