Add config step for unknown gamepads

This adds a quick config step for unknown and non-standard-mapped gamepads to let us figure out the right stick axes. It just prompts the user to hold left then up on the right stick, seems to work well in testing.

I added a list of known gamepads consisting of all the dual-analog gamepads I have laying around here that work on various platforms. I also made it save user-provided mappings using localforage (which uses IndexedDB or whatever as a storage backend). Ideally we'd have a web service for saving and retrieving mappings, but that feels like too much work.