Make peer assessment an optional step.

This also creates the AssessmentWorkflowStep model to track all
steps for new workflows. Some problems with it now:

* a lot of broken tests
* some display oddities on the peer step during (peer, self) workflows
* have not tested to see if there are concurrency issues with step
  model creation under load.
Updating tests and some simple cleanup.
Merge pull request #302 from edx/ormsbee/workflow_steps

Make peer assessment an optional step.
Supporting Peer Only configuration for ORA2
Merge pull request #311 from edx/sanchez/TIM-253-Peer-Only

Supporting Peer Only configuration for ORA2
Implemented autosave
Fixed a bug in which whitespace was added to saved responses on page reload.