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.

We throttle registrations by IP. We record IP address of each
registration, and if too many registrations have been done by the same
IP address in the time interval, we reject the registration.

I modified the SamplePlugin to show how to do some real processing,
adding a data class and an action class and modifying the main
menu to link to the new action. I added documentation comments to
all the methods and made sure the modules were PHPCS-clean.