Resolve "participant home screen - room logic"

Closes #42 (closed)

Proposed changes

• Added back button to participant-room view
• Added room logic so that the participant-room view displays current-room information instead of test data
• Added loading-spinner and error page for unavailable room

For testing

• Login as participant, join a room and check if you see the correct room-informations
• Click on the 'back'-button should go back to the participant-home-screen view
• By entering route to not existing room the app should show an error