Reimplement Special:Filepath as a redirect through Special:Redirect/File.

Reduces code duplication (and some ugly deprecated code patterns in the
Special:Filepath implementation).  In the long term, reduces the need for
one-off redirection pages (Special:PermanentLink, Special:Filepath, etc)
in favor of an extensible system based around Special:Redirect.

Change-Id: I42d5879342bc18412f0383c538e83c68b1c3cc2a
Merge "Reimplement Special:Filepath as a redirect through Special:Redirect/File."