Resolve "Connect FindGameController to backend"

Changes in this merge request

Frontend

• Add HTTP handling for creating, joining and leaving a lobby in FindGameController
• Add polling for lobby status through HTTP in FindGameController
• Add "Awaiting opponent..." popup window - Appears after pressing "Create Lobby" button
• Create HTTPRequestHandler class that improves upon ReceiverHandler class by adding retry functionality

Backend

• Split /lobby/:id/ endpoint into /lobby/create and /lobby/:id/join
• Add /lobby/:id/status endpoint
• Import json parsing functionality and refactor HTTP responses context to use json format
• Add isFull() method to Lobby class and interface

Important ❗

HTTPRequestHandler will replace ReceiverHandler, meaning that ReceiverHandler will be deprecated eventually. LoginController and LeaderboardController must therefore by refactored to account for this.

Closes #33 (closed)