Add an "Open in External Editor" keystroke to Differential

Summary: Ref T13515. See PHI1661. If a file is selected, add a keystroke to click the "Open in External Editor" link.

Test Plan: In Safari, Chrome, and Firefox: used "J" to select a file, then "\" to open it in an external editor. (In Safari and Chrome, this prompts.)

Maniphest Tasks: T13515

Differential Revision: https://secure.phabricator.com/D21135