ReturnAuthorization uses stock_location_id

- Replaced code in ReturnAuthorization#process_return to find the
  StockItem based on ReturnAuthorization#stock_location_id rather than the
  original shipment's stock location.
- Modified feature and controller specs to set the stock_location_id
- Added tests to the return authorization model specs that test
  returning to either the initial stock location or a new stock
  location. These currently fail (by design).