This is not a joke: The Joker sequel is a full-blown musical. The first trailer for Joker: Folie à Deuxis here, and it confirms that the film will feature a Joker (once again played by Oscar winner Joaquin Phoenix) with a song in his heart. He will be joined by Lady Gaga, who will play this Joker’s version of his star-crossed love, Harley Quinn. Also returning from the first film is Todd Phillips, Joker’s co-writer and director. The trailer shows Joker and Harley falling in love in a mental institution, and using music and dance to seemingly escape from the horrors of their incarceration. Half…