Move "External Editor" settings to a separate settings group

Summary:
Ref T13515. It's not intuitive that these settings are "Display Preferences", even thought they're intenrally related to some of the other display preferences.

Give them a separate group.

Test Plan: {F7370500}

Maniphest Tasks: T13515

Differential Revision: https://secure.phabricator.com/D21140