R1:45f347fe4777

Use EditEngine for diff, display, developer and most email settings

Summary: Ref T4103. This converts other straightforward panels to modern stuff.

Test Plan:
- Edited various settings.
- Tried to set a bogus editor value.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4103

Differential Revision: https://secure.phabricator.com/D16023