This introduces LocalSiteAwareModelFormMixin, which can be mixed into

any ModelForm that may need to deal with LocalSites. It handles

validation of any relation fields (such as Many-to-Many/Foreign Key

choice fields) to ensure that any values selected are compatible with

the form.
By default, forms are not tied to any LocalSite. They can create new

objects or edit existing ones, allowing the changing of a site, the same

way they do today. Based on the LocalSite field, though, the relation

fields will be validated to ensure they're on the same site, if a site

is chosen.
Forms can be limited to a particular LocalSite during construction. In

this mode, they can only create or edit instances for that site, and

relation values can only be chosen if they're bound to that site.
This ensures consistent, testable behavior that can be plugged into any

ModelForm. Future changes will begin to migrate some forms over to

this.

All unit tests passed.
Tested this with upcoming changes that convert forms over to using

this, and RBCommons usage of those forms.