Only show relevant people in kitchen crew info

[matthijskooijman]

Currently, there are two issues:

• Since we added the dinner preferences selection, everyone is shown in the kitchen crew info, most people with "no preference". These could just be filtered out, "no preference" is the default.
• A lot of people put "N/A", "None", "Geen", "Nvt" or variations thereof in the allergies box. We should either filter those out (either at save time strip them, or at display time), or change the UI of entering them to prevent this (maybe have a radio button with "I have no food allergies to take into account" or something like that).