Way to disable default Login mask

[APaskowski]

Is your feature request related to a problem? Please describe.

Right know if you add OIDC support the login dialog got bloated by default login and notification There should be a away to disable it.

Describe the solution you'd like

Add a setting (Bool) to disable default login mask. Maybe add a fallback url in case of a total failure.

Greetings

[nilmerg]

#5500 ?

[lippserd]

#5500 ?

This PR can serve as a basis, but the functionality should land in a separate PR. I think it makes sense to make this option non-configurable: If no authentication backend is configured but at least one LoginButtonHook is present, the default login form should be automatically disabled.

[Al2Klimov]

If you want OIDC-only logins, just remove all other auth backends. Depending on the system you might still want a local MySQL one with one emergency admin user.

If you're concerned about the UX, see:

• https://git.icinga.com/users/sign_in
• https://rt.icinga.com

Both have both forms and users are happy with it.