Renamed "Wikidata repository" special page group into "Wikibase repository"

"Wikidata" is just wrong since the extension itself is called "Wikibase". If this is an issue on
wikidata.org, then this should be solved there by changing the message only there.

Change-Id: I6a2775e7c6a0f8c4d60e3c6aac142a3a58d9a343
Merge "Renamed "Wikidata repository" special page group into "Wikibase repository""