Bump govuk_frontend_toolkit to 0.12.1

This is to make sure that we're picking up any major changes that have
occurred in the last few months.
Provide a specific id for the nav for when using horizontal navigation

This fixes a bug that was occurring against the main design principles
list as it was vertical rather than horizontal.
Strip out duplication of nav tags in views
Remove dodgy IE7 hack
Used versions for gems as per our development styleguide
Group gems by kind

They're now ordered:
  - External gems
  - GDS gems
  - assets
  - development/test
Setup asset pipeline to use SASS

The intention is to reduce all of the extra CSS that we appear to have gained.
Use the application*.css generators for different browsers
Use h2 instead of h3
Remove id from nav element as it's unnecessary
Refactor CSS to use SASS instead of ERB

This is a large refactor of the CSS to use SASS and the frontend
toolkit.

Most of this is fairly simple in terms of consolidating CSS blocks and
reducing duplication. The main thing to take note of is that all of
the mobile/media query CSS has been removed.
Update the examples boxes to fix the text/image alignment
Get styleguide nav working on mobile
Removing caption divs around text blocks

These weren't necessary and didn't add anything to the page.
Consolidate a lot of the 'example' classes

These classes were redefined multiple times to essentially do the same
thing. Reduce all of this duplication into a simpler block.
Make the feedback box visible on mobiles
Increase code font size to improve readability
Use the generated stylesheets for all of the browsers we generate for
No need for this include as it's handled in application.css.scss
Remove old IE classes that aren't used any longer