This updates RepositoryForm to use the new

LocalSiteAwareModelFormMixin, providing automatic validation of

related objects and the ability to limit choices to a LocalSite. This

simplifies a complex form just a bit, helping making it less likely that

a Local Site leak will occur down the road.
The largest part of this change is the introduction of new unit tests

that ensure that the form can be limited to a LocalSite without

problems. Otherwise, it's mostly about inheriting from the mixin,

removing old (redundant and slow) validation logic, and removing some

now unnecessary state tracking.

Unit tests pass.
Tested manually creating and modifying repositories, including changing

LocalSite associations.