Add FontAwesome as a font/icon choice

Summary:
This adds FontAwesome and attempts to make use as icons as consistent as possible. May require additional tweaks once we start using, but in practice this is pretty finished.

• Adds FontAwesome
• Adds additional transforms (rotates, spins)
• Adds additional colors
• Better scopes halflings and fontawesome
• Shares CSS between fonts for consistency

Test Plan:
Tested various browsers back to IE8, mobile.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8818