This makes a handful of changes for the hosting service registry, adding

typing, new documentation, and utility methods designed to replace those

in reviewboard.hostingsvcs.service.
HostingServiceRegistry now has convenience methods for retrieving or

unregistering a hosting service by ID. Logging from the methods in

service.py have also been added. With this, the service.py code is

now just a thin wrapper over the registry.
Pre-population of the registry via the initializing signal now happens

within reviewboard/hostingsvcs/__init__.py, which is guaranteed to be

run. It no longer relies on service.py being imported somewhere.

All unit tests pass.