[Refugees and migrants block a highway as they protest demanding the opening of the Greek-Macedonian border near the town of Polikastro]GENEVA (Reuters) - The top United Nations human rights official voiced concern on Thursday that a deal to manage migrant flows struck by the European Union and Turkey could lead to "collective expulsions" of refugees in violation of international law. Zeid Ra'ad Al Hussein said that there was "a contradiction at the heart of the agreement" between its stated aim to return all refugees and migrants and its assurances that individual claims for asylum would be assessed. ...