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 
This code creates a full dictionary for all the courses, although we require only one.  So even if the course we are looking for is first, we still loop through the rest.  Although it's a minor performance gain at this layer in the code, you can consider using the 'next' method as follows:

```
student_dict['cohort'] = next(
    (c.name for c in student.course_groups.all() if c.course_id == course_key), 
   "[unassigned]"    
)
```