In a summer of political and racial tumult, young Americans are in a dour mood: pessimistic about the fairness of their economic system, questioning the greatness of the United States and deeply skeptical of the way the nation picks its leaders.A new poll of young people between the ages of 18 and 30 finds that an overwhelming 90 percent think the two-party political system has real — though fixable — problems or that it is “seriously broken.” Three-quarters believe the U.S. is “falling behind …