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 
Instead of duplicating the test code, use `ddt` to test the various reports (for now: grades, students).  With that approach, you can also `ddt` the `test_calculate_grades_csv_already_running` method.