GD firebase register and login example

Registration with username, email, password

Login with email and password
User profile with delete account, logout and change password

Error notifications for
- not all fields are filled
- entered email has no email format
- entered usrname is already taken
- an account already exists with the entered email
- password is shorter than 6 characters
- login data was not correct
- Password has been changed

Send a verification email at registration (disabled in the demo)
Username max length
Username only allows A-Z, 0-9 and underscore
Create cloud database fields for registered users