RegistrationForm should use custom Application model

[jannon]

This is a fix for #274. All tests pass so unless the author of f5a1a49 can provide a failing test, I think the custom application model support should be restored. This is really a bug that completely breaks custom application model use in registration

[outsbart]

@jannon could you please provide a failing test reproducing #274?

[jannon]

Ah, knew I forgot something. Sure thing.

[jannon]

actually, we need only use the TestApplication model in the settings and the existing application registration test will catch the failure

[synasius]

This can't be merged because the get_application_model at module level raises the exception described in #151.
BTW, I pushed a proper solution which I'll merge asap. See #323