Order Quantity for Donation Tickets Missing and Donation Ticket of 0 USD should not be possible / minimum amount not working

[mariobehling]

1. Order Quantity for Donation Tickets is missing. Expected: Donation tickets could also be limited if e.g. the venue has limited space.

2. Donation ticket of 0 USD is possible even though minimum amount of 1 USD is defined in the ticket settings. In any case 0 should not be possible for a donation ticket ever.