MAPG-61 add onbeforeunload function to prevent accidental leave

public/static/js/mapguesser.js

@@ -470,4 +470,9 @@
     document.getElementById('startNewGameButton').onclick = function () {
         Core.startNewGame();
     }
+
+    window.onbeforeunload = function (e) {
+        e.preventDefault();
+        e.returnValue = '';
+    };
 })();