Basic authentication logic

I have added the basic logic to enable authentication.

- Login creates an authentication cookie
- Sessions are maintained via cookie authentication
- Logout or browser close deletes the cookie

Not yet implemented is refreshing the authentication key and cookie every N cookie authentications (to prevent session fixation).

There is also a trivial test of the authentication system in create_game.html, where the currently logged-in user's username is set automatically to be one of the players in a new game. There is no code as yet to deal with the case when a user is not currently logged in.