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 
This is the dashboard interface click handler, not the management command, but the management command can output a file with the same syntax.  Of the two other data download buttons which output csv's, one gives back a file which is templatized like this and the other gives back a statically-named file.  I considered this to be the better option of the two in case an instructor is running more than one class and wants to get this data for both of them without file name collisions.  As to file naming conventions in management commands, it seems like there isn't really a standard, so I just reused the file name from the dashboard.