LocalSite.objects.get_stats() now performs the total and public

LocalSite counts in one query instead of two, leveraging aggregate()

and Count() to get the totals. This can potentially be faster for

databases, allowing tables to be scanned just once instead of twice (if

needed). At worst, from testing, it doesn't appear to be any slower.
To ease some of this, we now have an index on LocalSite.public. This

can help databases determine a total without a scan.
LocalSite.objects.get_local_site_acl_stats() has also been optimized,

eliminating JOINs and instead using the through tables for user and

admin membership. This can make it much less expensive to get the IDs

from the database.

Successfully evolved the database.
Tested this query on SQLite and MySQL.
Unit tests pass.