Feature: disable password login

WP [#7662](https://www.openproject.org/work_packages/7662)

Requires merging of [this](https://github.com/finnlabs/openproject-self_registration_by_mail/pull/17) into `openproject-self_registration_by_mail`.
Do we really need the additional check here? If password login is disabled, there shouldn't be users having passwords, otherwise they can't login.