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 
Github displayed this diff in a really weird way. The `.data-display-table element` used to be a child to `.data-download-container`. With the UI reorganization, `.data-display-table element` now is a child to `.reports-download-container`. I removed the `.data-download-container` block, since there were no rules specified within it.