Create a Special:Redirect page.

Create a Special:Redirect page.

The primary purpose of this page is to redirect to a user page given a
numeric id.  The numeric User ID is stable across renames, and is
therefore an appropriate primary key for identifying the user
associated with a given revision.  The Parsoid API would like to
export semantic RDFa in its DOM identifying the author of a revision
by their userid, but in order to do so requires a MW redirect from
userid to the appropriate User page.  (A "permalink" for the user.)

This patch adds that redirect, as
  http://somewiki/Special:Redirect/user/1234

(https://bugzilla.wikimedia.org/show_bug.cgi?id=45206 is the related
Parsoid feature.)

Rather than adding a set of ad-hoc redirection pages, this patch
sets up an infrastructure for redirections.  Special:Redirect also
subsumes the functions of:

* Special:Filepath (Special:Redirect/file/xxxx)
* Special:PermanentLink (Special:Redirect/revision/xxxxx)

This structure is extensible for other redirect types.

Change-Id: I8b0785f4fbdb3dd438a7a45263c5f375ff9d2208