delete user

WP [#17654](https://community.openproject.org/work_packages/17654)

Requires merging of #2325
Unfortunately, user deletion is still implemented to be a lengthy process. Therefore delayed_job has to be triggered (see https://github.com/opf/openproject/blob/dev/app/controllers/users_controller.rb#L277).

This would however introduce another place which we would have to refactor if OpenProject is to become multi tenancy enabled. So the absolute correct way would be to create a [Job object](https://github.com/opf/openproject/blob/dev/app/controllers/users_controller.rb#L277)