Apply global notification settings changes to existing users

Notification settings applied at wiki level are only valid if the user didn't perform any custom notification choice. However we should allow admin to be able to automatically change settings for existing users with dedicated settings if they want.
That's especially true with XWIKI-17945 to apply automatically filters to existing users.
Now this should never be automatic: admin should be asked if they want their changes to be applied to all users. We should also consider that it could be lots of user so that the process to apply the change can take long.