Stop using borders and negative margins

The background to the navigation was previously some negative margin and
border-top fun. Change that to use divs with backgrounds. This means
that we don't need to add page overides in to remove the grey bar. It
also gives us future potential to try new header styles out.
Add in page navigation to html publications

This will need some more styles adding soon but I have added enough to
make it look not broken. There is a separate story for styling.

I also took the liberty of removing lots of the extra markup as it was
not needed.
Merge pull request #356 from alphagov/html-publiction-styling

Add in page navigation to html publications

https://www.pivotaltracker.com/story/show/46065773