Pass OutputPage to SkinTemplateGetLanguageLink hook.

Pass OutputPage to SkinTemplateGetLanguageLink hook.

Hook handlers should have access to the OutputPage object,
so they can use information stored there for generating
language links. This is needed to allow efficient generation
of "badges" by the Wikibase extension.

Change-Id: Ic479e2fa5cc3a4d663e478858caee2742e6805b9