Fallback to most used canteen if none is provided

[mammuth]

I guess most people have only one or two canteens they're visiting regularly. As a result it might make sense to store the canteens that were questioned. Maybe we should also count how often each canteen was accessed.

This is useful, because we could then automatically suggest this canteen in case the user forget to mention it and eg. just asked "What to eat today?".