Five years since her last album, Warrior, and three years since she became embroiled in a famous sexual-abuse lawsuit with her longtime producer “Dr. Luke” Gottwald, Kesha has released a long-promised new album. The 15-track Rainbow is a blatant, angry response to the singer’s battle with a legal system that has left her feeling frustrated and trapped as an artist—but also a powerful pop album that earns the anticipation.