Fix ext-content parsing to handle non-html5, non-extension tags.

* Commit 0b8840d6 implemented hierarchical parsing for extension
  content.  But, it didn't correctly handle non-html5 tags like
  <big>, etc. that aren't really extensions but valid html tags,
  just not html5.

* Fixed tokenizer to handle this.

* Fixes RTing of snippets like <big>[[Foo]]</big> (TODO: add test)
  Eliminates rt regressions on these (and other pages) all of which
  seem to be using <big> tag:
  - en:Arabical
  - en:Hurricane Ivan tornado outbreak

* One wt2wt failure -- it looks like the earlier greening of the
  "<tag><tag></tag></tag>" was a side-effect of a bug.  The failure
  looks like a bug elsewhere which is independent of these patches.

Change-Id: I2d3d5daccedd41f7a0fe3b923f48b3addc81ffb7