index.html, src/{game,simulate}.js: Properly seed each game.
- Seed each new game individually, not each session.
- Print the seed to the console to allow reproduction of individual games.
- Allow recreating games by giving seed to newGame() command.
- Move global prng from simulate.js to proper global game state "Game".
- Improve ToyPrng's validation of new seeds.
src/{game,intro}.js: add intro video to UX