Add Special:ListDuplicatedFiles expensive query special page.

Add Special:ListDuplicatedFiles expensive query special page.

I saw some comments recently on commons suggesting that
better ways are needed to manage duplicate files (There are tools
for if a specific file is a duplicate, but no backlog of
outstanding duplicate files).

This seems like a fairly easy first step in that direction.

Wasn't sure if this should be an image gallery type
query page, or just a list. I think in this case a plain
list is more useful.

Change-Id: Ibe4b9da71ca6451ec4e6b0050feaf3ca70e1b888