Implement heading-24 mixin
Implement heading-24 mixin

Block of styles used core-24 mixin but matched the format of heading-24
so replace.

robinwhittleton/static,tadast/static,kalleth/static,alphagov/static,robinwhittleton/static,kalleth/static,kalleth/static,robinwhittleton/static,tadast/static,tadast/static,kalleth/static,alphagov/static,alphagov/static,tadast/static,robinwhittleton/static