User registration and login only with e-mail address in Drupal 8

At the beginning of the "Krampus" weekend, which became famous in the rest of the world thanks to the movie, I show you, how to realize hiding the username and only use e-mail address in user registration and login instead in Drupal 8. We had to accomplish this yesterday in a current Drupal 8 project. Having only a few hours left to delivery date and still some open issues to solve, we needed a quick solution....