empires: Rename all files (html, js, and css) to game.*

empires: Rename all files (html, js, and css) to game.*

These are all the files for manipulating a single instance of a
game. We're moving this away from index.html so that we can have a
separate page at index.html for giving game instructions and allowing
users to host a new game.

So, along with the new name of game.html, we also rename supporting
files to game.css and game.js to be consistent.