It’s haunting and mournful and hopeful and beautiful. It’s called “Ashokan Farewell,” and it’s the de facto theme song for the Ken Burns miniseries The Civil War, which premiered 25 years ago this week. I mean, just listen: “Ashokan Farewell” was not, as both its tune and the miniseries that made it famous would seem to suggest, written in the 19th century.