Use WikitextContent to render redirects

Core change Id44d566a moves redirect header rendering into
WikitextContent, removing the need for extensions to special-case
redirects in order to call Article::viewRedirect to get the proper
output.

Bug: 14323
Change-Id: I3653b608941813a73281f4f0545bea2487d43964
Depends-On: Id44d566a7ca35a1b9579d0c0e947877c980b0686
Merge "Use WikitextContent to render redirects"