Add Vector and Monobook skins to main Wikimedia extensions

The skins have messages to translate and their functionality
is pretty much the same as of the extensions.
Follows up on 9779b5db

Change-Id: Ifb25894628c860dc6d4af45b8b7268e0d533d6da
Merge "Add Vector and Monobook skins to main Wikimedia extensions"