Show disabled input for Service Desk custom email with help text when not configured

Proposal

In the Project Settings > General > Service Desk UI, make the custom suffix section visible, but with a disabled input and a suggestion to configure service_desk_email when the user does not have service_desk_email configured.

Background

Currently users are confused about the service_desk_email configuration section. Many think it's required for setting up Service Desk in general, when that's done using the incoming_email section, and wonder why Service Desk isn't working properly when it's configured.

Similarly, when they do have incoming_email configured (but not service_desk_email) they don't see the custom email address portion of the UI. It is simply hidden from view with no guidance or help.

This change makes it visible and points the user to the documentation where they can learn more. Combined with the documentation update in #295640 (closed) this should make Service Desk configuration clearer.