A potential showdown looms next month between comedian Kathy Griffin and President Donald Trump. Griffin is set to attend the White House Correspondents Dinner on April 28, according to an announcement she made via Twitter. There, she may have her first face-to-face moment with the President since her infamous “severed head” photo of him hit social media. Griffin’s career has nosedived since she posed deadpan last year with a bloody severed head that resembled…