This introduces a siteconfig_reloaded signal, which consumers can

listen to in order to update their state when a SiteConfiguration has

expired and has been reloaded. The signal contains the new instance,

along with the old instance for settings comparison purposes.
A couple of changes were made to enable use of this signal.
First, the behavior of SiteConfigurationManager.check_expired has

slightly changed. Previously, expiration just meant clearing the value

from cache, causing the next get_current() to re-fetch. Now, if there

are any listeners for the signal, the new instance is fetched

immediately, so that it can be passed to the signal handler. This

shouldn't impact any consumers. If no listeners are present, the new

instance is not fetched.
A new method get_for_site_id() method was also added for use in

check_expired(). This works like get_current(), but takes the ID of

a Django Site, handling caching. get_current() wraps this.
The docs throughout the file have been modernized, helping to keep this

maintainable going forward.

All unit tests for Djblets and Review Board pass.