After multiple permutations of “The Karate Kid” — including the 1980s movies, a gender switch and a 2010 remake — the original players are reunited in “Cobra Kai,” a new series designed to kick up attention for YouTube Red. For the most part, it’s an extremely clever revival, an overachiever that knowingly plays off the title’s kitsch factor while evolving the characters and augmenting the cast in savvy ways.
