New design, deprecated placeholder classes in favour of mixins

DO NOT MERGE as this still hasn't been polished by the design team. We'll look into it on Monday. Please comment fixes to current implementation
Alright then, I'd be curious to see how many products use this. Looking forward to meet other teams to see what they actually use.

Also: demos are one source of documentation for the default classes, but what about documenting classes available by default when linking to the buttons in the build service?