“If you keep your head down, he’ll move on and bother somebody else,” 16-year-old Vivian advises the new girl at school who has been bullied by a particularly odious classmate. “Thanks for the advice,” comes the response, “but I’m going to keep my head up. High.” That brief exchange is enough to ignite Vivian’s feminist awakening in “Moxie,” a sincere if ultimately empty coming-of-ager ...