WIP: Workflow API Integration for AI

@wedaly @gradyward 

Still cleaning up tests, but this should cover the AI integration into the Workflow API, such that:

* AI will always be the first step, and immediately submit the essay for grading, and move onto the next step, when the workflow is created.
* The grade will not be generated until all steps are completed.
* AI score will be used if Peer and Self are not present.

Also if you look at the workflow/api.py, the change in the signature to create_workflow() bothers me. I'm tempted to change the need for the "rubric" and "algorithm_id" to "assessment_requirements", which would be more in line with the other functions. Any thoughts? 
Pinging @ormsbee if you have time / thoughts on this.