Implement a passwordless or even usernameless flow

[brynwhyman]

For FIDO2 it's possible to implement a passwordless or even usernameless flow using Resident Keys (see https://developers.yubico.com/WebAuthn/WebAuthn_Developer_Guide/Resident_Keys.html).

Should this be built, for people who'd want to utilise this functionality, we'd want to make a note that it would be good practice for them to enable 'User Verification'. I've raised an issue to disable it by default in #68, but it enabling it for this use case would make sense.

[robbieaverill]

Game changer! Send it!