Update GOV.UK frontend toolkit from 0.46.1 to 1.2

This is a big jump in version numbers but not such a big jump in
code changes.

https://github.com/alphagov/govuk_frontend_toolkit/compare/17aa587bbb578c4a551e56d39467c8fbda7482c7...9248f820e8a416070928d16fd71260198c6c681d

- Error colours now exist
- Improved CSS mixins
- Standard GOV.UK icons are now available
- Three quarters variable
Use new scale() mixin from frontend toolkit

Because shared code FTW.
Merge pull request #531 from alphagov/update-govuk-frontend-toolkit

Update GOV.UK frontend toolkit