Support standalone puzzles without an associated hunt

[matthew-zhang-306]

Writing puzzles is fun, but sometimes writing an entire hunt feels daunting. I'd love to be able to share individual puzzles that I design without having a full puzzle hunt in mind. We can create hunts with one puzzle in them for this purpose, but that feels bulky and workaround-y - it would be great if standalone puzzles were supported as a first-class concept