Implement new service for creating user

app/controllers/admin/users_controller.rb

app/controllers/registrations_controller.rb

app/models/user.rb

app/services/users/create_service.rb

...gelogs/unreleased/27878-new-service-for-creating-user.yml

doc/api/v3_to_v4.md

lib/api/users.rb

lib/api/v3/users.rb

lib/gitlab/o_auth/user.rb

spec/controllers/registrations_controller_spec.rb

spec/models/hooks/system_hook_spec.rb

spec/models/user_spec.rb

spec/requests/api/v3/users_spec.rb

spec/services/users/create_service_spec.rb