Allow WantedQueryPage subclasses to override the existence check

Some pages are still "wanted" even if the page exists. For
example, consider a page in the file namespace that exists but
doesn't have a corresponding file, in the context of
special:wantedfiles.

Change-Id: Ie40f2baea22b3564e7f06c4fa21bc1efdc0e4d54
Better false positive detection for Special:Wantedfiles

Previously the page crossed out entries based on Title::isKnown.
This could potentially catch pages which are valid file redirects,
which shouldn't be crossed out.

This shouldn't affect anything performance wise, as the previous
Title::isKnown check would call wfFindFile anyways.

Change-Id: Ia9eac8f5618181c631a5b2b1c461b12a90f2dde7
Merge "Allow WantedQueryPage subclasses to override the existence check"
Merge "Better false positive detection for Special:Wantedfiles"