This introduces User.get_local_site_stats(), which provides a list of

IDs of LocalSites that the user is a member of, and LocalSites that the

user is an admin of, along with a state_uuid that can be used as part

of derived cache keys.
This enables quick lookup of LocalSite membership. The results are

cached, making it quick to call, saving up two database queries,

depending on the usage. The cache key makes use of the state_uuid from

LocalSite.objects.get_stats(), meaning that the cache will

automatically be invalidated any time the list of LocalSites change.
User.is_admin_for_user() makes use of this, benefiting from the cache.

The old version of this function did have a cached copy, but it didn't

correctly invalidate.

Unit tests pass.