Refactor frontend layouts, add html-version layout

The html-version is going to look slightly different from everything
else we have at the moment. The easiest way of doing that is to have its
own layout.

Refactored the other layouts so that they all share the same wrapper and
only change out the content of the whitehall-wrapper.
Inital styling of html publications

There is talk of moving these out of the static template at some point
but for MVP this will do.
Add optional numbered headings in govspeak

As per the desired design we want the headings on the page to be
numbered. Made it so that the heading number level is configurable on a
per-page use.
Add test exclusion for html-pub layout
Style html publications for a chromeless wrapper

Added a method of switching slimmer template for individual controllers.
@h-lame assisted as my inital attempts didn't work due to the way before
and after filters work.

Added a standalone html-publication stylesheet with its ie variants.

Styled with the assitance of @markhurrell. It uses very minimal styles
form elsewhere as it doesn't have the nta font and is typeset in
helvetica.
Add new Home Office detailed guidance categories

https://www.pivotaltracker.com/story/show/46806237
Merge pull request #371 from alphagov/html-pubs

Style HTML Publications

https://www.pivotaltracker.com/story/show/46066201