President Obama was the crooner-in-chief Thursday night, taking the stage at Harlem’s famed Apollo Theater and serenading supporters with an Al Green classic. Capping three fund-raisers in the city, Obama softly sang the opening to “Let’s Stay Together,” with Green himself giving the Prez props for his chops. “We’re so honored. It’s the first time we’ve ever had a sitting President at the Apollo,” the venue’s CEO, Jonelle Procope, said of the event where tickets went for $100 to $25,000. Earlier, Obama met Jewish leaders at upper East Side restaurant Daniel and said the election is “going to be a...