This introduces a new manager stats method,

LocalSiteManager.get_local_site_acl_stats(). This returns a dictionary

containing:

The number of users on the Local Site
The number of admins on the Local Site
Whether the Local Site is public
A UUID representing the generated state (which can be used as a

   component of other cache keys to enable automatic invalidation)

This is per-Local Site. It exists on the manager so that either a

pre-fetched LocalSite instance can be provided, or the primary key of

a LocalSite. That makes it usable in methods where we may not yet want

to fetch a LocalSite.
This information is cached. It's invalidated whenever user/admin

membership changes, when public does or may have changed (based on

whether LocalSite.save() is called with/without update_fields=), or

when a LocalSite is deleted.
This will soon be used by other query-generating code to conditionally

fetch data from the database, and as components for other cache keys.

Unit tests pass.