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 
Ideally, the methods in this file should be renamed as they are not specific to S3 (since they use the ReportStore abstraction).  In the (hopefully near) future, we may support other data storage and this code shouldn't need to change.