Disable setting to prevent admin confusion

1. What is your original issue/pain point you want to solve?
   I wanted to give the customer the option to set the notifications.
   After some testing i checked google and found Customer cannot change and see his notification settings · Issue #1959 · zammad/zammad · GitHub
   I red it and understand it.

2. Which are one or two concrete situations where this problem hurts the most?
   Time loose because the settings clearly confuse and let the admin think it can give the notification settings to the customer.

3. What is your expectation/what do you want to achieve?
   Disable it or grayout so it is obious that you cant select/use it.

Your Zammad environment: 6.3

• What roles/people are involved: customer