Add SkinTemplateGetLanguageLink hook

Added SkinTemplateGetLanguageLink hook to allow changing the html of
language links. This is needed by the Wikibase extension to display badges
of sitelinks.

Bug: 60717
Change-Id: I223bf7a7bb0d13942a81b1766acbee62e080348d
Merge "Add SkinTemplateGetLanguageLink hook"