Leaderboard (updated with changes from master)

Updated version of #466 that:

* Resolves merge conflicts with master.
* Adds additional test cases
* Uses a version of `edx-submissions` that gets top scores from the read-replica and caches the results.
* Simplifies our XML tests so that the ddt test cases can check a subset of XBlock fields.

@stephensanchez Please review.
We may want an upper limit for the leaderboard somewhere.