Model and submit API updates

NOTE: Not ready for merge because tests are incomplete, but I need to stop for the day, so sharing progress.

Changes to model and submit API that make submitting straightforward. Submission is the most common action, so I rewrote the model such that submitting doesn't need to change User or Assignment records.

Per Stephen's suggestions, all API code is back in api.py and the NDB interface is an object. Comments are welcome.

Testing is incomplete, but I won't get to finish it until tomorrow. 
Also, why are we storing submitter separate from the parent? Doesn't the parent already have this information? Or could they be different in certain cases.