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 
@dmitchell The `ReportStore` can be configured to either (1) upload files to S3 or (2) store files in the local file system.  The string being generated here is to be used for the file name.

I created a JIRA ticket in the Course Reruns epic regarding this: 
https://openedx.atlassian.net/browse/LMS-11492