Resolve "Adapt frontend find game to #36"

frontend/core/src/com/game/tankwars/Callback.java

frontend/core/src/com/game/tankwars/HTTPRequestHandler.java

frontend/core/src/com/game/tankwars/ReceiverHandler.java

.../src/com/game/tankwars/controller/FindGameController.java

...c/com/game/tankwars/controller/LeaderboardController.java

...ore/src/com/game/tankwars/controller/LoginController.java

frontend/core/src/com/game/tankwars/model/CurrentUser.java

frontend/core/src/com/game/tankwars/view/FindGameScreen.java