Add spacing css

Summary: Adds ability to add 20, 10 and 5 px of margin or padding without adding additional rules.

Test Plan: Reloaded any Phabricator Page. It didn't break.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5477