Getting replays out of web Neverball

[parasti]

https://neverball.github.io/

Finally added a way to get replays out of the Emscripten filesystem. A save button appears in the Replays screen that acts as a download link for the selected replay.

I had to modify the screen UX a little - it used to select replays via hover, but that breaks down for multiple actions (watch replay + save replay). Now you click once to select, click again to play, and I also added a play button because the save button was lonely. (Typing this I realize gamepad UX probably suffered.)

Some part of it is really slow on mobile (maybe FS.readFile), but it works if you wait long enough.