Implement checkmate, resignation and draw

[dev-xero]

Implement a system for terminating active games. It must signal both players about the game state.

Acceptance criteria

• Must notify both players when checkmate, resignation, or draw occurs.
• Must update the game model with the appropriate reason.
• The game state should be persisted in the main database.
• Update player ratings based on their elo difference.

Notes

• Would probably need to store a shouldUpdate flag in local storage to fetch the new ratings after a game.

[dev-xero]

Add support for timeout too, through socket messages.