Fix for TIM-449. Do not use Peer UUID

Conflicts:
	apps/openassessment/assessment/peer_api.py
Do not use the submission_uuid from a response for peer
The Workflow API was still using the submission_uuid within a peer response.  Removed this, and updated tests accordingly.
Merge branch 'release'

Conflicts:
	apps/openassessment/assessment/peer_api.py
	apps/openassessment/assessment/test/test_peer.py
	apps/openassessment/xblock/test/test_peer.py