Stop PO number from duplicating when copying event

Would close #256

RIGS/rigboard.py

@@ -121,6 +121,7 @@ class EventDuplicate(EventUpdate):
         old = super(EventDuplicate, self).get_object(queryset) # Get the object (the event you're duplicating)
         new = copy.copy(old) # Make a copy of the object in memory
         new.based_on = old # Make the new event based on the old event
+        new.purchase_order = None
 
         if self.request.method in ('POST', 'PUT'): # This only happens on save (otherwise items won't display in editor)
             new.pk = None # This means a new event will be created on save, and all items will be re-created