Support multiple WebAuthn authenticators per user

[marvinruder]

Is your feature request related to a problem? Please describe.
Users may have multiple WebAuthn clients like iCloud Keychain, Yubikeys etc., but currently cannot register more authenticators or move to a different one, i.e., they are bound to the one used during registration.

Describe the solution you’d like
Users can view, add, rename and delete their WebAuthn authenticators in their Profile Settings. At least one authenticator must be present at all times.