Adding functionality for Remove Submission from grading pool.

This would maintain the history for deleted submissions via new database table.
TNL-900
This scenario would not work. 
User B maybe able to see the User A's essay but when he tries to submit the response, an error would be raised  in _get_server_and_client_submission_uuids method in peer_assess handler in peer_assessment_mixin.py. 

I'm adding the test for this.