(bug 43912) Special page group for Wikibase repo

Implemented only for repo, with intention of separating these and
yet-to-come client extensions in two groups, bacause repo and client are
used separately.

Change-Id: Iad2b3e145cc20a34d4fbffd6bcd5e0969056ed39
Merge "(bug 43912) Special page group for Wikibase repo"