UI: Fallback Admin 2FA / App Passwords

[mwinters0]

Currently, the 2FA and App Passwords UIs for fallback admins gives no warning that you cannot use these features as a fallback admin. Subsequently, you will likely add the QR code to your 2FA app before hitting submit, only to find that this is not supported.

These items should stay in the sidebar to avoid confusion, but clicking them should just yield a small "This feature is not supported for fallback admins" warning page with a link to https://stalw.art/docs/auth/authorization/administrator/#fallback-administrator .