Fix FontAwesome fonts in Firefox

Summary: Fixes T7983. Moves FontAwesome to only affect the :before attribute, which all icons are based on.

Test Plan: Tested Chrome, Safari, Firefox on Mac. Chrome, Firefox, IE on Windows.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7983

Differential Revision: https://secure.phabricator.com/D12635