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 
My opinion is:  `course_id` > `unicode(course_key)` > `course_key.to_deprecated_string()`

But Jim seems to not prefer `course_id` in this case.