Issue #2002162 by klausi, eugene.ilyin: Convert form validation of users to entity validation.

core/lib/Drupal/Core/Entity/EntityStorageControllerBase.php

core/modules/user/lib/Drupal/user/Entity/User.php

...upal/user/Plugin/Validation/Constraint/UserMailUnique.php

.../user/Plugin/Validation/Constraint/UserNameConstraint.php

...gin/Validation/Constraint/UserNameConstraintValidator.php

...upal/user/Plugin/Validation/Constraint/UserNameUnique.php

...user/Plugin/Validation/Constraint/UserUniqueValidator.php

...modules/user/lib/Drupal/user/Tests/UserValidationTest.php

core/modules/user/user.module