WASHINGTON — They came from a place of heartbreak to claim their spot in history: hundreds of thousands of teenagers and supporters, rallying across the United States for tougher laws to fight gun violence. The “March for Our Lives” events on Saturday drew massive crowds in cities across the country, the kind of numbers seen during the Vietnam War era. In Washington, D.C., New York City, Denver, Los Angeles and other cities, demonstrators heard from student survivors of last month’s school shooting in Parkland, Florida. “If you listen real close, you can hear the people in power shaking,” Parkland survivor David Hogg said to roars from protesters packing Pennsylvania Avenue from a stage near the Capitol to a spot near the White House many blocks away.