bind leavegame to this

static/script.js

@@ -269,6 +269,7 @@ window.addEventListener("DOMContentLoaded", function() {
             this._onKeyDown = this._onKeyDown.bind(this);
             this._onKeyUp = this._onKeyUp.bind(this);
             this._onSendClick = this._onSendClick.bind(this);
+            this._leaveGame = this._leaveGame.bind(this);
         }