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).  
A convention we've followed elsewhere is to pass in the rubric dictionary from the problem definition, then use `rubric_from_dict` to get-or-create the `Rubric` model.  This would allow you to retrieve a leaderboard even if the student had not yet made a submission.