Give IconSetControl a meaningful disabled state

Summary: Ref T10004. This control doesn't disable visually or behaviorally, e.g. when locked in an EditEngine configuration.

Test Plan:

• Locked field for Projects.
• Reviewed form in EditEngine.
• Created/edited a project.
• Swapped default.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10004

Differential Revision: https://secure.phabricator.com/D14911