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 #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.

app/assets/javascripts/application.js

@@ -1,16 +1,11 @@
+//= require header-footer-only
+
 //= require libs/jquery/jquery-ui-1.8.16.custom.min
-//= require libs/jquery/plugins/jquery.base64
 //= require libs/jquery/plugins/jquery.player.min
 //= require libs/suchi/isOld.js
 //= require govuk_toolkit
-//= require user-satisfaction-survey
-//= require core
-//= require report-a-problem
 //= require geo-locator
 //= require jquery.history
 //= require jquery.tabs
 //= require mobile
 //= require browser-check
-//= require analytics
-//= require vendor/polyfills/bind
-//= require govuk/selection-buttons

app/assets/stylesheets/application.scss

@@ -1,31 +1,22 @@
 /* govuk_frontend_toolkit includes */
-@import "colours";
-@import "conditionals";
-@import "css3";
-@import "device-pixels";
-@import "grid_layout";
-@import "measurements";
-@import "typography";
+@import "header-footer-only";
+
 @import "shims";
 @import "design-patterns/buttons";
 
 /* local styleguide includes */
 @import "styleguide/conditionals2";
 
 /* styles */
-@import "helpers/alpha-label";
 @import "helpers/buttons";
 @import "helpers/core";
 @import "helpers/ancillary-pages";
-@import "helpers/beta-label";
 @import "helpers/footer";
 @import "helpers/header";
 @import "helpers/multi-step";
 @import "helpers/notifications";
 @import "helpers/organisations";
 @import "helpers/publisher";
-@import "helpers/report-a-problem";
 @import "helpers/text";
-@import "govuk-component/component";
 
 @import "static-pages/error-pages";