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!
@sarina I agree that the bare except is strange and not usually a good idea.  However, here what I want to do is update the pending tasks table to reflect a failure if data collection fails regardless of error type.  It is not impossible to catch a specific set of exceptions here, but I do not think that that is what I want to do, since I cannot anticipate every way that the database connection might fail mid-task and would like to update the UI to reflect failure irrespective of the cause.  I will switch it to your suggestion with broad-except for now, but if there is any better way to handle this situation, I would be happy to do that instead.