Ran through the codebase with pyflakes. It caught one missing
ImproperlyConfigured import.

There's no security issue with checking local URLs.

The to_python logic of the field was not actually executed as the fields
meta class wasn't set. That's also the reason why AbstractAddress
explicitly called upper() again, which is redundant now that it works.

This is better handled using the model's clean method.

This is necessary for South migrations. As it doesn't define any new
parameters, an empty ruleset is all that is needed.

Plus reorganise tests

Please note that ExtendedURLFields's verify_exist does not
default to True anymore.

There is now a model field, a form field and a validator.