This introduces LocalSite.objects.build_q(), which builds a Q()

result based on the provided LocalSite/LocalSite.ALL value and

whether Local Site functionality is used in the deployment.
If either LocalSite.ALL is passed or Local Sites are not in use, the

result will be an empty Q(), which will be optimized out when joining

with another.
If Local Sites are used, this will construct a Q(local_site=...)

value.
It also provides validation on arguments, ensuring that a LocalSite

instance cannot be accepted if the database does not contain any

Local Sites. This will mostly help us with writing correct tests and

to catch errors with stale caches.
This will greatly help us build queries in the most optimized way

possible. On normal deployments without Local Sites, these will be left

off entirely, allowing the database to make more optimized query plans.

Future changes will begin to introduce usage of this method.

Successfully made use of this in some in-progress changes, and

verified with those unit tests.