Change to use govuk_frontend_toolkit box-sizing mixin

as per @alextea's comment
https://github.com/alphagov/static/commit/35c97f054f45f48cbbcc6e93f735cf
b3a263a51d#commitcomment-3264149
Using govuk_frontend_toolkit border-radius and -box-shadow mixins

as per @alextea's comment
https://github.com/alphagov/static/commit/35c97f054f45f48cbbcc6e93f735cf
b3a263a51d#commitcomment-3264157
now using "image-url" to refer to image in the stylesheet

no idea what this means/does, am just following @alextea's suggestion
(works fine on my VM)
https://github.com/alphagov/static/commit/35c97f054f45f48cbbcc6e93f735cf
b3a263a51d#commitcomment-3264334