Force admins to use new voucher system

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We now have a voucher system such that teachers cannot see the whole user database anymore. As risen as concern in this comment, we might want to force admins to use this new voucher system as well during

• Lecture editor assignment
• Lecture teacher assignment
• Talk speaker assignment

In other places, e.g. the admin user dashboard, admins can still the whole user base. However, as admins are also teachers, we might want to use the voucher system for them as well such that students have the same experience in all lectures/seminars (and not some where this voucher system is magically not needed).

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To find these places, search for placeholder: t("basics.enter_two_letters").