First version of the separation of book_ahead into create_ahead and delete_ahead, allowing policies to distinguish between creating new bookings and editing or deleting existing ones.   Still to do:  test PostgreSQL; adjust draggable bookings code to recognise bookings that can't be edited; and add messages when you try and delete a booking that can't be (at the moment the booking is just silently not deleted)

git-svn-id: http://svn.code.sf.net/p/mrbs/code/mrbs/branches/create_policy@2877 25f7e783-145e-4f2d-a9f8-dd7182bfe9c7