Add FIDO U2F/WebAuthn hardware auth

[rugk]

As the request came up in mastodon/mastodon#562 maybe you can add WebAuthn so one can login with a hardware dongle such as NitroKey or YubiKey.

GitHub can be taken as an example, where the UX of this is really nice. (E.g. automatically trigger at login and so on)

[nightpool]

it looks like https://github.com/CyberDeck/devise-fido-u2f already has some u2f code, although i don't know how well the libraries play together.

[LeeteqXV]

PS. It should ideally be implemented so multiple registrations per account is possible.
Ref./FYI: "Multiple 2FA registrations per account" (TOTP + U2F)
mastodon/mastodon#10149