Redo sass (bring in the good work from drop)

(https://github.com/HubSpot/drop/commit/24104c94bfe0f42212b0ad40c82d5d7df3412d35, https://github.com/HubSpot/drop/commit/c91f671d41c7ce865cb4f18bfb8f578a2ef8dce5, etc.)