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 
Since this file doesn't ever reuse the constants TEST_COURSE_ORG, etc, consider removing them and just using the default arguments to CourseFactory.  It will create random course key information for the tests.