Replace deprecated configuration variable $wgSpecialPageGroups

With the getGroupName() method overridden for these classes:
* Wikibase\Client\Specials\SpecialUnconnectedPages
  the only Client special page
* Wikibase\Repo\Specials\SpecialMyLanguageFallbackChain
  the only Repo special page not extending SpecialWikibasePage
* Wikibase\Repo\Specials\SpecialWikibasePage
  the base class for almost every other Repo special page

Change-Id: Id7fbaac189b1c5ccbf32fe420864c5326c9b03b5
Merge "Replace deprecated configuration variable $wgSpecialPageGroups"