Reworked/more explicit guidance on use of cookies

Include information about different types of cookies and their intrusiveness. Add rules on what information must be published about cookies and how to notify users about them.

This change adds some new rules for services that should probably be reviewed more widely before merging such as:
- services should include their own cookie information page (with links back to the GOV.UK edition)
- services should notify users about cookies using the blue banner style notice (even if GOV.UK has notified users already) because the cookies are likely to be different to those on GOV.UK.