WebBasket: 'move item' improvements

* Adds a warning when moving fails due to duplicates
  in target basket.

* 'move item' now works if a new basket is created
  during the move.

Signed-off-by: Krzysztof Lis <krzysztof.lis@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>