WIP: Data went in, but doesn't any more

* remaining items:
 - replace submission fk w/ m2m on peerworkflowitem
 - complete move
 - initialize feedback box, grayed out
 - tests

* TODO: incomplete move, usage still in submissions api
Changed some models and apis around. Some small tweaks to the UI. Migration.
TIM-231: Addressing feedback from @wedaly

* Improved error handling and reporting
Ensuring the right many to many relationship is preserved for Assessment Feedback