President Biden was heckled on the White House lawn Monday by gun control advocate Manuel Oliver, whose 17-year-old son Joaquin was killed in a 2018 mass shooting at a high school in Parkland, Fla. “Sit down! You’ll hear what I have to say,” Biden said as Oliver shouted over him during a celebration of bipartisan gun policy legislation that Biden signed last month. Oliver’s remarks weren’t clearly audible on pooled video feeds, but Biden pushed back and defended his record on gun control. “We have one. Let me finish my comments,” Biden said to Oliver, before quickly adding, “Let him...