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 wasn't sure that we wanted to couple our naming conventions with the CSV upload process, although based on our conversation with Ed last night on the importance of keeping our naming conventions consistent, I'd also be fine with including that logic in the `upload_csv_to_report_store` helper.