(Bug 43994) New Special:UnconnectedPages

This special page "Special:UnconnectedPages" uses a page prop "wikibase_item"
that holds the id for the item at the repository.

Later it could be other tables that are better suited, and updated faster,
than the page_props table.

The actual page prop in use is defined in
https://gerrit.wikimedia.org/r/#/c/51179/

Patchset 3: Rebased to use the last additions for page props
Patchset 4: Tryed out Daniels proposal
Patchset 5: Rebase to fix an error due to old changes
Patchset 6: Fix of bug introduced by "JOIN" should be "LEFT JOIN"
            Also fix of whitespace (carriage return).
Patchset 7: More cleanup
Patchset 8: More cleanup - where do those carriage return comes from??

Change-Id: I5cfab43ee8c7feff79c8b4bda0183142dc3e2c98
Merge "(Bug 43994) New Special:UnconnectedPages"