TIM-548
Bugfix: 500 error when clicking "peer assess" with no submission.
Quiet noisy error log message
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.
TIM-499: Database error in peer_assess handler uses variable before assignment
Merge pull request #308 from edx/will/no-open-assessments-error

Quiet noisy error log message
Merge pull request #307 from edx/will/status-details-error

TIM-548
Merge pull request #310 from edx/will/TIM-499

TIM-499: undeclared variable when logging an error
Updating tests and some simple cleanup.
Merge pull request #302 from edx/ormsbee/workflow_steps

Make peer assessment an optional step.
Update wording on grade complete page