Authentication_Screen

• implemented in AuthenticationScreen using custom LoginWidget
• LoginWidget can switch between 3 different modes:
  • reset mode (isReset is false): User can reset password for an already existing mail adress
  • login mode (isReset is false, isLogin is true): User can login with registered credentials
  • and signUp mode (isReset is false, isLogin is false): User can enter Email and Password to create a new account
• the methods signIn, signUp and sendPassWortResetEmail implement the respective authentication functions from Firebase (https://firebase.google.com/docs/auth/flutter/start)