Will/grady/self assessment feedback

@wedaly , we should talk about the Test changes and the changes to models/base.py
This test is too closely tied to the implementation.  At this level, the tests shouldn't know that the implementation is using a `RubricIndex`; it also shouldn't be calling private methods of `AssessmentPart`.

It would be better to call `create_assessment()` with different values of `feedback` and `overall_feedback` to simulate the error, then verify that the expected exception is raised.