Refactor customizable key mapping

[bfirsh]

From review in #136:

• Did you consider lifting the state up? I think this might make the implmentation a bit neater. keyboardController.keys could be passed to ControlsModal. You could then have a setKey method on KeyboardController that was passed all the way down to each row. That way the state management and business logic is all in KeyboardController.
• Is there a reason why it is persisted on unload rather than on every key change? I would have thought setting local storage is pretty cheap. That way you wouldn't have to check if state were modified and the keys would still persist if unload wasn't called for whatever reason.