This was moved to a core preference as part of T285402
The migration was handled by a #user-notice
Bug: T285402
Change-Id: I25fe3d32ad929924034db06b5a95bab6a3a3a727
For consistency with other skins, its easier to see how hooks are
invoked when they are separated from the skin.
The skin is also updated to use namespaces and modern skin registration
which has been available since MediaWiki v1.36.
Change-Id: I1c93d4e817b53c93eba47ee9086c86c5d75443e5