Changed models to KeyProperty from StructuredProperty. Fixed tests.

So it turns out that StructureProperties aren't actually stored in the database....

It's not an actual reference to another entity. Look at [this](https://developers.google.com/appengine/docs/python/ndb/properties#structured), and in particular the paragraph below the code.

I switched the code to use `KeyProperty` instead of `StructuredProperty`. We can still access the referenced entities using `submission.submitter.get()`, for example.