Jack Dorsey, Twitter’s chief executive, was working remotely on a private island in French Polynesia frequented by celebrities escaping the paparazzi when a phone call interrupted him on Jan. 6. On the line was Vijaya Gadde, Twitter’s top lawyer and safety expert, with an update from the real world. She said she and other company executives had decided to lock President Trump’s account, temporarily, to prevent him from posting statements that might provoke more violence.