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).  
Leftover assertion?