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 
The `to_deprecated_string` method is deprecated.  :)  Instead, you can do `unicode(course_key)`.

But what would be really better (instead of an extra conversion back) is renaming course_id to course_id_string so it is clear what type it is.  However, that would require changing the URL API and may be outside the scope of this PR.  All URL handlers in Studio, for example, call them as `course_key_string`.