Ora2 aggregated data download button and management command

This is a pull request for an Ora2 aggregated data download button.  It is located in the Data Download section of the Instructor tab in edX.  It runs a raw SQL query located in instructor.utils (formerly located here https://gist.github.com/wedaly/f17ba1d2fffae65545c8) against the database.  This request also includes an associated management command, which performs the same query against the database, and can save the output file to an optionally specified directory or print it to stdout.

It would be nice if you guys could take a look at it and make suggestions before I attempt the upstream PR.  @jrbl @stvstnfrd @jbau @kluo 
I was taught to let library functions parameterize sql queries rather than doing it oneself to avoid injection attacks.  I admit that that's an unlikely scenario here, since the course_id isn't specified by a user unless they already have permissions to run management commands, but it seems safer to do it this way.  I also think it's more readable - the difference between cursor.execute(query) and cursor.execute(query, [params]) seems self evident to me especially given the conditional, but that is subjective.