This introduces LocalSiteManager, which is the new manager for

LocalSite objects. It provides methods used to quickly make

determinations about the Local Site usage on the server.
The main method currently is get_stats(), which returns a dictionary

containing:

The total number of Local Sites on the server
The number of public Local Sites
The number of private Local Sites
A UUID representing the generated state (which can be used as a

   component of other cache keys to enable automatic invalidation)

This is cached and invalidated any time a LocalSite is created,

deleted, or if the public state has changed, making it fast and

reliable for lookups.
There are additional wrappers around this that leverage the cached

state. These include:

has_local_sites()
has_public_local_sites()
has_private_local_sites()

These will soon be used to make some parts of our queries conditional on

their presence, letting us simplify queries in the common case.

Unit tests pass.