[WIP] Background-ify 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.

@nasthagiri 
I believe this error is at the XHR level, therefore I think the message is appropriate (though @lamagnifica can verify). If the error was `AlreadyRunning`, then the API call would have returned `JsonResponse`, hit the `success` callback, and displayed the message the server provided.