Well, at the very least, President Harding’s wife is a rock star. The powerhouse vocals of Richelle Howie as Florence Harding are a highlight of Landless Theater’s lively but uneven production of the musical President Harding Is a Rock Star, an ironic tribute to a man widely considered one of the worst presidents in American history. And while Ms. Harding may well have been the force that got the incompetent Warren through two years of…