Don't warn about conflicts when reverting Phriction documents

Summary:
Fixes T5144. This was incorrectly checking the content version, not the head version, so reverts would raise the "conflict" warning.

Also fix a couple of FontAwesome icons.

Test Plan:

• Edited a document.
• Reverted a document.
• Opened two edit tabs. Edited one, tried to edit #2, got a warning.
• Opened two revert tabs. Reverted in one, tried to revert in #2, got a warning.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5144

Differential Revision: https://secure.phabricator.com/D9249