Before Joss Whedon become public enemy #1 for comic book movie fans after 2017’s “Justice League,” he was a television wunderkind. “Buffy The Vampire Slayer” remains the popular fantasy TV show of all time. Its spin-off, “Angel,” also had a devoted following, and the cult love for Whedon’s “Firefly” is unparalleled for a show that lasted just nine episodes.