Don't allow "Conpherence" menu items to be added to editable menus if Conpherence is not installed

Summary: Depends on D20336. Ref T13272. Fixes T12745. If you uninstall Conpherence, "Edit Favorites" and other editable menu interfaces still allow you to add "Conpherence" items. Prevent this.

Test Plan:

• Installed Conpherence: Saw option to add a "Conpherence" link when editing favorites menu.
• Uninstalled Conpherence: No more option.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13272, T12745

Differential Revision: https://secure.phabricator.com/D20337