Remove ApplicationChangeRequest and ApplicationUserData models

[Anthony Lenton]

On our road to splitting out devportal from sca, we can now remove the ApplicationUserData and ApplicationChangeRequest models, as the published json is now stored on the Application itself.

It might be a good point to review our change review workflow: We will need more states for "published apps with changes in review", but we might want to add a "published" bit that effectively duplicates all states, so that updates can go through the exact same workflow as the initial submission.