Setting additional metadata for completing a peer workflow

Completes TIM-227 by making a set of peer workflow items declared as "scored", so we always know which assessments were used to generated a peer workflow score. 

Includes migration
Includes splitting up API calls in self and peer to obtain assessments.
Tests updated.

Added Serializers for PeerWorkflow, PeerWorkflowItem, which I did not end up needing, but intend to use later. I can pull this out into a separate PR if needed. 

@ormsbee @wedaly