Dual authentication

[KoenVanduffel]

Right now several authentication methods are available but no dual authentication is possible. On the roadmap there is the "OAuth/OIDC SSO Replacement (Authelia, Authentik, etc.) [#344]" on the short term list, looking forward to use that!

Without knowing what short term means another maybe easier / quicker to implement thing is to require both username + password ánd email / OTP which is not quite the same security profile as using 2 different services but comes very close. This might also be very easy to set-up for users as it could be just a tick mark to require both forms of authentication.

@milo (I see you are the core maintainer) just pointed out that the user can setup dual authentication. Being able to force that from admin side would achieve the same as the above.