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.
Wow, also I really don't like https://www.gov.uk/guidance/style-guide/a-to-z-of-gov-uk-style as a piece of content. Again, suggest writing-for-gov-uk is the preferred resource.