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