Update links to the GOV.UK style guide

The GOV.UK style guide is now part of the new [content guidance](https://insidegovuk.blog.gov.uk/2014/10/07/content-guidance-now-in-one-place/).

The mappings have been done by the Depts & Policy content team.
Merge pull request #487 from alphagov/update-style-guide-links

Update links to the GOV.UK style guide