Adding functionality for Remove Submission from grading pool.

This would maintain the history for deleted submissions via new database table.
TNL-900
It's great that the big workflow tests include this, but I'd like some tests explicitly for cancellation.
Examples:
1) Student submits essay
2) workflow is cancelled
3) What happens to the student?

1) Student submits essay
2) Essay cancelled
3) Peer looks for essay (finds nothing)

1) Student submits essay
2) Student picks up peer essay
3) Student essay is cancelled
4) What happens to the peer, and the open assessment?

1) Student submits essay
2) Student essay cancelled
3) Student tries to get another peer essay for review
4) What happens?

Just some examples