'admin' is a pretty common username that people try when installing;
it was blacklisted because all of our admin panels were at /admin/*,
which would conflict with the admin user's namespace.

Changed the location of all admin panels to /panel/*, blacklisted the
nickname 'panel', and allowed 'admin'. Tested with a fresh install;
seems to work great.

Only show local public notices in sitemap. Only do counts for them
in the sitemap index, and only show them in the notice sitemap.

We need to bundle counts of notices and users by date. This can be
expensive for large sites. So, new tables are added to cache the
results of these queries, which don't change after the date is over.