Asynchronous download button for ORA2 data

This pull request implements a management command and instructor dashboard button which gather all open assessment response data for a given course in a user friendly format and output it as a .csv.  It does so by submitting a task to the celery queue and waiting for it to upload the result to S3 in an asynchronous fashion.  Data collection is performed via a templatized, raw SQL query (admittedly a bit unorthodox), the original version of which may be found <a href="https://gist.github.com/wedaly/f17ba1d2fffae65545c8">here</a>.  Also included are unit tests for the functions added in various locations.  Feedback is welcome and encouraged @jbau @sarina as well as tagging more people who should have a look.  Thanks!
@dylanrhodes: you mentioned switching to a `202 Accepted` status code -- I think that's the more accurate HTTP status code to use, both for this and for the existing asynchronous grader. We're trying to focus more on developing reusable APIs within our codebase, and as a result I'd like to continuously work towards better interfaces in our codebase. My only hesitation is that I don't know if there is Javascript that requires that the status code be exactly 200 -- I doubt it, but it's possible. I'd suggest that you try using the 202 status code and testing it out. If it works, keep it; if there's a Javascript failure that takes more than 5 minutes to debug, then skip it and we'll deal with it in a future pull request. Is that fair?