Editor's Note: (((Newsweek))) is fake news. Newsweek reporter Kurt Eichenwald said Tuesday that his baseless claim that President-elect Donald Trump was once institutionalized in a mental hospital was actually part of a series of jokes and intended to be a "signal to a source" to talk to him. Eichenwald's explanation came Tuesday on "Good Morning America," as host George Stephanopoulos asked him about the September tweet in which he said, without evidence, he believed "Trump was institutionalized in a mental hospital for a nervous breakdown in 1990." "Any regrets about that?" Stephanopoulos asked. "There's a long story behind it," Eichenwald said after some brief laughter. "When you go through the full lead-up to that tweet — there was a reporting purpose for that tweet going out, which is…