Allow wikipedia subdomains in Special:ItemByTitle.

This is a preliminary solution for supporting wikipedia subdomains
(instead of global wiki ids) for identifying a target wiki in
Special:ItemByTitle. This allows Special:ItemByTitle to be used directly
as the target of a rewrite rule covering xyz.wikidata.org.

Bug: 45080
Change-Id: I51aefa44ef8d05773d83dae2d36a32f44265339e