If the Marvel Cinematic Universe is a rich, diverse tapestry of superhero films that all connect in one way or another, the Avengers movies are the tentpoles that glue the whole thing together. Getting to Marvel’s The Avengers was a herculean task unto itself, but following that up with a second Avengers movie—crafting a reason for the heroes to come together again while also ensuring it feels different and fresh—was a completely different kind of challenge. Avengers: Age of Ultron is a weird, dark, sometimes messy sequel, but it wears its ambition on its sleeve and takes …