Drop django-registration app from Pootle codebase

Seems that the code was not on Pootle codebase because it was necessary to
localize it, but perhaps because this way was easier to release Pootle.

The new registration app doesn't save the RegistrationForm, so we can't
rely on it for saving the agreements accepted when the user registered.

Fixes bug #2579
Drop django-contact-form from Pootle codebase

Since this app was kept for localization a custom app that extends
django-contact-form was put in place just to keep the form localized.

Fixes bug #3139