This updates both GroupForm and DefaultReviewerForm to use the new

LocalSiteAwareModelFormMixin, providing automatic validation of

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

simplifies the forms a bit and expands the testing and security of the

forms.
Error messages have also been updated for the various related fields,

providing more useful, contextual errors. Future changes will help make

these even more consistent across forms.

Unit tests pass.