FontAwesome RemarkupBar

Summary:
Replaces the icons with fonts from FontAwesome. Up in the air about the meme icon. Thoughts?

Also removed the second fullscreen/normal state. Seems obvious what it does, but assume someone complained previously?

Test Plan:
Tested all the icon states and made sure they still worked. Test fullscreen and help.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9385