Bump GOV.UK template from 0.7.1 to 0.8.0

https://github.com/alphagov/govuk_template/compare/v0.7.1...v0.8.0

- Moves cookie message above header
- Fixes HTML-escaping of asset path using Mustache
- Switch to using the new global colour bar
Merge pull request #500 from alphagov/update-govuk-template-0-8-0

Bump GOV.UK template from 0.7.1 to 0.8.0