Add govuk_component template and styles

For use as a shared partial which can be loaded by any application.

- Add new route and controller action for govuk_component templates
  Separate from the existing templates so it is clear as to what is for
  use as a component
- Use a elements sass file for all elements styling so it can be easily
  included between both the header_footer_only stylesheet and the
  application stylesheet
- Include beta-label as an example component
- Bump the toolkit for the new beta label styling
Merge pull request #430 from alphagov/element-partials

Add govuk_component template and styles