Process HTML <pre>...</pre> as a single, self-contained token

 * Avoids the nested <pre> issue in the PreHandler.

 * Cleans up a bit of the ParagraphWrapper and tries to separate behaviour
   of hasOpenPTag and hasOpenHTMLPTag.

 * Adds a test for a pre tag nested in a block.

 * Fixes an issue with the normalizer where nested spans that would
   normally be ignored weren't. This results in a new passing test.

 * This was suggested in:
	https://gerrit.wikimedia.org/r/#/c/96678/

 * Relevant commits:
	20c6afe7f51d14e978f348f376d58e28a9d2b3df
	81ab0f9f1d9a5a0830f1d14a98e10a959d173620
	a2025499347494de36ca20570c74d3604d06c627

Change-Id: Id786604fe04196ee2744f238d1dbbd2efc0cbb05