Book multiple rooms simultaneously

[CHTJonas]

Enhancement to add the ability to book multiple rooms with the same booking, for example to simultaneously book the stage and dressing rooms during a Get In. This could be implemented as follows:

1. Feature to clone a booking from one room to another, thereby creating a new booking model identical to the previous except in that it references a different room model.
2. Feature to book multiple rooms at once from a single booking, thereby creating a many-to-many relationship between the booking and room models.