The mixin for ensuring Local Site restrictions for forms was

double-patching querysets when limiting to a Local Site. The first patch

would happen at form construction time, and the second would happen at

form cleaning time (though this one was temporary). This caused queries

like SELECT ... WHERE local_site_id=1 AND local_site_id=1.
This change fixes that to only patch during form cleaning if it hasn't

already patched at form construction.
It also updates the field accesses to work with newer versions of

Django, and splits a unit test for these restrictions into two to avoid

an ordering difference on the newer versions.

Unit tests pass on Django 1.6 and 1.11 (with other in-progress changes).