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!
I agree that it would be more portable to use the ORM, but we did explicitly scope this story as "take the query we're running manually and automate it". Once the new ORA team is ramped up, they could ORM-ize this when we next want to change the format, or someone in the community can do it if they want this functionality to work with a different DB.  (@explorerleslie: fyi).