Decode entities in nowiki and pre contexts as well.

* <nowiki>&lt;</nowiki> renders as "<" in the browser in the PHP
  parser which means we ought to decode these as well.  Same for
  <pre>&lt;</pre> as well.  Failure to decode them in the tokenizer
  caused RT errors on these snippets.

* Unrelated fix: the regexp for escaping entities in Util was
  missing the "/g" flag.

* Fixed the tokenizer to recognize html entities which fixes the
  parse and RTing of these snippets.

* Fixes some RT errors on en:ASCII Art Farts

* No change in parser tests (TODO: Add more parser tests).

Change-Id: I7522e8f3a27a218cc0ae4c15d75fd877390e6ded