Article styleguide

Basic style guide to show base styles, components, layout and an article page with all the elements we are currently using in the alpha.
Refactored govuk_static css into components and base css
Added sass linting


We have duplication here. "The Pension Guidance Service" is common to all pages so could be moved to the application layout. "Style guide: " is common to all style guide pages so could be moved to the styleguide layout?

Additionally, our controller is "styleguide" (no underscore) so I'd expect us to use the word "Styleguide". If we're using two words "Style guide" our controller should be named "style_guide_controller.rb".