If there was any doubt, the uproar this week over BuzzFeed’s publication of unverified allegations about President-elect Donald J. Trump made clear that the gatekeeper role once played by major news media organizations has vanished in the digital age.This poses a deep danger for legitimate, aggressive journalism, especially from the president-elect, who has been consistent in his heavy-handed demonization of any and all media whenever he dislikes critical but accurate stories about him.BuzzFeed’s irresponsible decision to publish a seamy and wholly unsubstantiated research dossier about Mr. Trump by a former British spy gave him the opportunity to attack not only...