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.