Revert "Revert "Merge branch 'design-principles'""

This reverts commit f483c5668903d2467a2b0ac6d474dd1764e1c1b1.

Provide a branch on which to continue those changes.
Whitespace, indentation and closing tags
changed FAQs and added links
Set up path for page & add images
Re-size images to fit grid and make responsive
Correct filename for SEO template
Corrections to markup
Improvements for non-visual users
Add service manual redirects
Service manual updates #version-18

https://github.com/alphagov/government-service-design-manual/compare/version-17...version-18
Merge pull request #35 from alphagov/design-principles

Design principles