Adding functionality for Remove Submission from grading pool.

This would maintain the history for deleted submissions via new database table.
TNL-900
Curious if this scenario works:
1) User A submits essay
2) User B pulls User A essay for review
3) User A's essay is cancelled.
4) User B submits an assessment for User A's essay

I think this still works, but I'd like a test in test_peer.py to explicitly define this scenario