registration: Support conditionally registered namespaces

registration: Support conditionally registered namespaces

Some extensions (e.g. EventLogging) conditionally register a namespace
depending upon the wiki's configuration using the CanonicalNamespaces
hook. Since the "namespaces" key automatically registers the namespace,
we need an option to not register it and let the extension take care of
that manually using the hook.

All of the other namespace related settings are safe to unconditionally
set since they won't do anything if the namespace doesn't actually
exist.

Bug: T141604
Change-Id: Ie8c217fdc8bd783b30f98210309ea56fef39c0da