Move "Open in Editor" to File Box

Summary: Moves this single action to the File Contents box in Diffusion Browse. Also fixes a PHUIObjectBox missing when enable highlighting is on.

Test Plan: Enable/Disable Highlighting. See disabled Editor button.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4467

Differential Revision: https://secure.phabricator.com/D8300