Add option to set minimum amount of tickets for event

**Is your feature request related to a problem? Please describe.**
I use this app to do bookings for training sessions I give. My sessions, however, have a requirement of a minimum 5 attendees in order to proceed as planned. Currently, it's only possible to set a maximum number of seats for an event, but not a maximum.

**Describe the solution you'd like**
I would like to be able to specify a minimum number of seats as well as a maximum. Also have the app show the number of currently booked seats.

**Describe alternatives you've considered**
I've considered putting the minimum requirement in the event description. However, most of the time people will miss this. Also, there's no way to provide people with an overview of how many seats are already booked (which may influence their choice for a date).

**Additional context**
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