OSS project E804: A way to query db models to return scores, without UI changes

2 new classes participant_scores.rb and assignment_scores.rb have been added wherein the scores are returned in one database query. Database views have been used to optimize the time. The file scores.rb has been changed to make use of database views for querying. Appropriate migrations and models have been added. 

The return type of scores.rb has been changed in order to minimize a level in the multilevel hashes used at many places. Deprecated code has been removed and replaced.