“Mamma Mia! Here We Go Again,” the perfectly titled sequel to “Mamma Mia!” (it opens 10 years to the week after the first film), kicks off on a bubbly high. It’s 1979, and Donna, the free-spirited expatriate-on-a-Greek-island innkeeper played by Meryl Streep, is now played, at the end of her Oxford undergraduate days, by Lily […]