Allow more than one reservation for the same period

[developeregrem]

Currently one apartment can have exactly one reservation for one period. There are use cases where it would be great to allow more than one reservation. For example, a bigger room might be occupied by different people that do not belong together but share the same room. To enable this a room should become a new property multiple occurrences. If this property is set to true it is allowed to add more than one reservation for the same period if the max. capacity of the room is not reached. The reservation overview must show these new reservations accordingly.