Add -unconnectedpages to pagegenerators

Since now Wikibase client does support API wrapper around
Special:UnconnectedPages (commits: 6f037b08, ff02f539, bd4d1e07 in
mediawiki/extensions/Wikibase) and it's deployed, it would be really
good to have such a generator.

Bug: T56738
Change-Id: I6b91bd4cf6ceaf5cd71889a81a639a88a8e8d117