More than 40 years after her burial in an unmarked Philadelphia grave, Sister Rosetta Tharpe, gospel’s first superstar and its most celebrated crossover figure, is enjoying a burst of Internet celebrity. A video of her playing one of her signature tunes, “Didn’t It Rain,” from a 1964 TV special filmed for British television has been racking up more than 10 million views on YouTube and Facebook. Old and new fans the world over, dazzled by Tharpe’s powerful singing and wildly …