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
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.