[Redesign] Try fa-code icon for open Diffs

Summary: Ref T8099, changes the icon from open to 'code'. Feels slightly better.

Test Plan: btrahan, epriestley

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D13055