Return item cancellation

Creating a new return item should cancel any other return items with the same inventory unit.
This ensures an inventory unit can only be active in a single RMA at a time.
This allows history to be preserved but still allow new RMAs to be created to replace old ones.
@athal7 yeah, exactly.  My approach was that it's ok to have cancelled or awaiting items already because we're going to ignore or cancel them, respectively.  It's just if we had one that was flagged as received or given_to_customer that would be a problem.  Shouldn't ever happen except maybe via stale HTML pages that get posted, etc.