Move redirect rendering into WikitextContent

There's no good reason for everything that wants to render a page to
have to test whether the page is a redirect and then call
Article::viewRedirect to get the fancy rendering instead of using the
ParserOutput. This logic can easily be moved into
WikitextContent::getParserOutput so callers can just use the
returned ParserOutput.

At the same time, we can handle "#REDIRECT [[File:Foo]]" and
"#REDIRECT [[Category:Foo]]" the way people expect, by recording the
link in pagelinks rather than imagelinks/categorylinks (although this
means fixing ImagePage's bug about assuming anything in imagelinks that
is a redirect is a redirect to the image too).

And we can finally fix bug 14323, too.

Bug: 14323
Bug: 17259
Bug: 27621
Bug: 42642
Bug: 50488
Change-Id: Id44d566a7ca35a1b9579d0c0e947877c980b0686
Followup: I1c7582d1bf7ec4184a45b00154e3dd5b39dd444b
Followup: I3653b608941813a73281f4f0545bea2487d43964
Merge "Move redirect rendering into WikitextContent"