Refactor SCSS to use frontend toolkit for measurements

As per https://github.com/alphagov/hmrc-contacts/commit/f186ee458e85acd84263680fe5d2df423ecdcf88
I'm using the frontend tookit and only keeping that around for
measurements. There's also a PR at https://github.com/alphagov/govuk_frontend_toolkit/pull/110
for adding 3/4 into the toolkit.