feat(ui): add main menu navbar and join game functionality

knockoutwhistweb/app/logic/game/GameLobby.scala

@@ -70,6 +70,9 @@ class GameLobby private(
     if (!sessionOpt.get.host) {
       throw new NotHostException("Only the host can start the game!")
     }
+    if (logic.getCurrentState != Lobby) {
+      throw new IllegalStateException("The game has already started!")
+    }
     val playerNamesList = ListBuffer[AbstractPlayer]()
     users.values.foreach { player =>
       playerNamesList += PlayerFactory.createPlayer(player.name, player.id, HUMAN)