Strip U+0000 in wikitext
U+0000 is not allowed in HTML5, there's no reason to allow it in wikitext.
It simplifies our code if we can just strip them at the start. Strip in
PST as well so they don't sneak into our database either.
Tweaked the EXT_LINK URLs to account for the fact that invalid characters
get transformed into U+FFFD when using Preprocessor_DOM. See 73649741ed1e
(r65967) for context on that change.