PARIS — Left-wing European leaders tried to forge a common front Saturday in the continent’s migrant crisis while right-wing and extreme right groups protested current policies toward the waves of people fleeing war and poverty. The meetings and demonstrations came as thousands of migrants have massed in muddy camps in the Greek border town of Idomeni after countries across the Balkans closed their borders. Europe’s migrant crisis is forming the backdrop for key elections in three German states today.