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.