Group bookings: Individual assignment of extras per room.

When adding extra items in a group booking, we need to have the option to activate the individual allocation for each room.

For example, in a group of 10 rooms that contract breakfast as an extra, currently the most convenient is to add the 10 breakfasts in a room of the group, however, if we are asked to bill individually, it forces us to edit the entire reservation and add the item one by one for each room.