Rewrite basic layout to not use FET grid helpers

- Use my own simpler grid helpers
- Improve spacing/groupding of fixtures
Code highlighting with Prism
Model Components and Fixtures for easier template use
Split preview into partial, for easier reuse
List components on the front page, it's much more useful
nicer title/header/nav style
highlight around the component previews to make them more visible
Partial for component call example
Bugfixes

- grid helpers overriding wrapper top/bottom padding
- inconsistent preview/code order
- don't show locals if fixture is empty
Merge pull request #4 from alphagov/redesign

Redesign and refactor UI