Integrate Nitrokey support to set up & enable system-wide hardware-based two-factor authentication

What it says on the tin!
This issue is thematically well suited to the following discussions:
elementary/pantheon-agent-polkit#33
elementary/pantheon-agent-polkit#40
elementary/greeter#230
elementary/mail#345
elementary/files#862
https://github.com/elementary/switchboard-plug-onlineaccounts/issues/89
https://github.com/elementary/appcenter/issues/936

Unlike a Yubikey, it is Free Hardware and Free Software, which is mostly manufactured locally (in Berlin, Germany).

Here you can find more general information: https://www.nitrokey.com/ & https://github.com/nitrokey

This integration can also be used to decrypt your hard disks - see LUKS/LUKS2 - or as a solution for Two-factor authentication in Switchboard Online Accounts Plug. Integration with Files (to sign, encrypt and decrypt files) or Mail (to sign, encrypt and decrypt emails) and for installing Software with AppCenter it's also very useful.