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 
Abridge by using the imperative:
```Query aggregated open assessment data, write to .csv```

Python (and Django) have tools to automatically parse the structure of code blocks and do nice things to present docstrings, but they rely on a first-line-summary-blankline-explanatory-body format. So try to get in the habit of writing docstrings whose first line is a complete, imperative, summary. (When practical.)