Update to newer style of styling and template

- Use the header-footer-only template, this doesn't need the styles or
  JavaScript for the mainstream formats.
- Update govuk_frontend_toolkit for the new grid-layout mixins
- Use govuk_frontend_toolkit mixins for a global wrapper width
- Move content_for tags to the top of the template, so it is clear when
  you look at a template how that template effects the parent template
- Move the page specific sass to a partial Sass file (partial files have
  leading underscores) and put it in a view folder so its scope is
  obvious.
Add byebug for debugging in development
Move browse over from frontend and style

- Moved over the functionality from the browse controller on frontend
- Created views for those pages and styled as per designs
- Copied over the tests from frontend and refactored them to work with
  the new code. Added a couple of cucumber tests to hit parts the
  controller test didn't.
Improve display of browse on small screens

- Reduce the padding, remove the border.
- Added titles to the pages
- Put some padding around the detailed guidance section
Whitespace.
Set slimmer headers for analytics tracking

The version of slimmer in the project didn't correctly output the
analytics tags for the browse sections. Bumping the version to the
current version made the correct analytics tags get sent.

Also copied over previously missed dummy artefact from the root browse
page.
Merge pull request #7 from alphagov/new-browse

Move browse over from frontend and use new pattern

https://www.pivotaltracker.com/story/show/72441304
https://www.pivotaltracker.com/story/show/72441234