LocalSiteAwareModelFormMixin has been updated to work with a wider

variety of forms. Specifically, those that take in a request

parameter, which previously was not being passed in to the parent form.

Since there's no reliable way to introspect whether the parent requires

this parameter, and since the mixin itself needs it, we now have a flag

that the form can set to specify it needs this parameter. This will be

required for the IntegrationConfigForm in a future change.
This also improves the field support. ConditionFields are now

supported, and will have their LocalSite set in order to limit valid

results to those on the selected site. On top of this, the logic for

overriding and then restoring the queryset on fields has been expanded

to do the same with the local_site_name attribute on a field's

RelatedUserWidget, ensuring everything is back to normal after

validation if not limiting the form to a LocalSite.

Unit tests pass, both as of this change and with the upcoming work to

move IntegrationConfigForm over to this mixin.
Manually tested the forms with this mixin.