Parse #REDIRECT constructs in wikitext.

We parse the #REDIRECT construct and its variants:

 #REDIRECT [[Main Page]]
 #REDIRECT: [[Main Page]]
 #REDIRECT : [[Main%20Page]]
 #REDIRECT [[Main Page|piped link]]
 #REDIRECT [[Category:Foo]]  (which puts the redirect page in the category)
 #REDIRECT [[:Category:Foo]] (which does not)
 #REDIRECT [[File:Foo.jpg]]  (links to the file, does not embed image)
 #REDIRECT [[Image:Foo.jpg]]  (ditto)
 #REDIRECT [[simple:Main Page]] (not a language link; 'Simple:Main Page')
 #REDIRECT [[:simple:Main Page]] (not an interwiki link; same as previous)

gwicke: fixed small tokenizer bug

Bug: 45808
Change-Id: I1bd36f32a5e46b90261895e5499a0308875e5e05