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!
@singingwolfboy just seconding @shnayder here. Not having this query automated causes huge pain not only for the Stanford team but also internally for edX...we have this crazy devops process that gets this data for clients and PMs send manually. This is an important step toward the right direction, that takes a query that Will Daly wrote and automates it, even if it's not the final, optimal solution.  

cc @jinpa