Migrate U2F keys to WebAuthn

[mcguffin]

This is a proof of concept and a follow-up for #427.

The transformation is happening in includes/WebAuthn/class-webauthn-key-migrator.php. The PHP is mainly gathered together from sjinks/wp-two-factor-provider-webauthn and madwizard-org/webauthn-server.

Apart from reviewing, here's what I think should be the next steps:

• Key migration is currently triggered by the user in the U2F keys admin table. However we could do it automatically, eg. when the user is visiting his/her profile page, like sjinks/wp-two-factor-provider-webauthn does. I'm feeling indecisive about it and I'm looking forward for your feedback and opinions.
• Add an admin message after a key has been migrated.
• Real life testing with actual U2F / YubiKeys
• Unit tests

[burnedfaceless]

@dd32 Can you please merge this pull request for me? I would really like to be able to use my U2F Keys again.

[dd32]

dd32 Can you please merge this pull request for me?

This isn't my call. I would suggest using a plugin that adds a WebAuthN provider in the meantime: https://wordpress.org/plugins/two-factor-provider-webauthn/
(edit: Worth mentioning; I haven't reviewed that plugin, I know others who use it, I don't know if the future built-in Two Factor WebAuthN support will support it or migrate those registered keys)