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 
When you kick off a task that takes a non-trivial amount of time, an in-progress task table is rendered at the bottom of the dashboard. I think that's where this shows up. It seems like the past tense verb should be consistent with other elements of the UI. So maybe, "generated" would be more appropriate. Perhaps we'd then want to also rename the button from "Download profile information as a CSV" to something like "Generate profile information CSV"?