The CSS used to grey out the checkbox and label for
advanced IADS should be refactored so it can be reused for
disabling player checkboxes in the flight edit/create dialogs.
The solution is applied to all checkboxes/labels, so that
there's a consistent way of showing users when a
checkbox/label is disabled.
(cherry picked from commit d9c6be0a9d232f73269da720bf7c0f4dfa1b9329)