Issue #2002162 by klausi, eugene.ilyin: Convert form validation of users to entity validation.
Issue #2057869 by dawehner, jibran: Provide an alias for 'plugin.manager.entity' called 'entity.manager'.
Revert "Issue #2002162 by klausi, eugene.ilyin: Convert form validation of users to entity validation."

This reverts commit 5b41b6f82f0f27eee43a717dd495768f3e8c843c.
Issue #2002162 by klausi, eugene.ilyin: Convert form validation of users to entity validation.