Student Training (API and data model)

First pass at defining the API and XML format for Student Training.  The public API is implemented and tested, but not optimized.  An example of the XML format is included, but not yet implemented.

There was a small no-op migration for feedback text (setting the max length, which the database should ignore for that field type) that I guess didn't get included when the model was changed.  I've included it as a separate migration, before the student training model changes.

@stephensanchez