Make application CSS/JS extend header_footer_only CSS/JS

All of the CSS/JS modules included in the application 'layout' are
also included in header_footer_only - and only those modules - so
this makes application share exactly those modules by importing
header footer only.

This helps avoid bugs like https://github.com/alphagov/static/pull/458

Also by treating each layout as an extension of an existing layout,
layering new features on top, then we can easily add new kinds
of layout in the future.
Merge pull request #461 from alphagov/layout-inheritance

Make application CSS/JS extend header_footer_only CSS/JS