Change user title fallbacks to use id instead of username

[jsangmeister]

With OpenSlides/openslides-autoupdate-service#892, the restrictions are changed in such a way that the username of a user is only visible with user.can_see_sensitive_data. Therefore, the currently implemented fallback of displaying the username when a user does not have a first or last name will no longer work. The fallback should be changed to simply display User {id} instead. In real instances, almost all users have a first or last name anyway, so this will not really cause any changes for existing users.