Refactor header Sass into header.scss
Move tagline outside of h1

Keeping it in the header element, but moving it outside of the h1
is better for accessibility.
Fold 'copy' into 'tagline'

It doesn't seem worth it to have two separate fields in Stagecraft
(copy and tagline) that are both styled almost identically.

Add a new 'with-related-pages' class to control the width of the tagline.
Merge pull request #133 from alphagov/61978992-improve-page-heading-accessibility

Improve page heading accessibility