Leaderboard within ORA2 block

This pull request allows course developers to add a tag into their ORA2 construction
<leaderboard top="10" /> 
which will display a leaderboard below the students score of the top results for the piece of assessment.  

The leaderboard is added as a mixin, with another API added to the openassessment API.  All submissions are found using the rubric_id within the students submission which there may be an easier / less dodgy way however it looks to be the only way (apart from creating another table to store the leaderboard which may be beneficial in the long run).  
I'm concerned about the performance of these queries for a large number of assessments.

1) This iterates through every available assessment, which for a large course could be thousands or tens of thousands of records.

2) For each assessment, it performs additional database queries to retrieve the submission info and to calculate the number of points (`assessment.points_earned` is a property)

A more scalable approach might use the `Score` model in the `edx-submissions` API.  Since the `Score` model has a foreign key to ` StudentItem`, you could filter for scores that match a particular course/item.  Additionally, `Score` has a `points_earned` field that is written to the database (not calculated), so you could query that field directly.

@ormsbee may have some additional thoughts on this.