[FEATURE REQUEST] - add FIDO-U2F as optional 2nd factor

[user4andre]

This suggestion is primarily a supplement or alternative to #52.

If unlocking with fingerprint plus password (2 factor login) is implemented, FIDO-U2F could serve as a backup in case fingerprint recognition fails or as stand-alone alternative in case the sensor is missing.

If FIDO-U2F is implemented without fingerprint unlocking, it should be possible to register 2 FIDO-U2F keys to unlock in case of loss of one FIDO-Key.

[user4andre]

New fingerprint implementation works fine for me. I created this request as alternative solution before knowing that fingerprint will work that good.

Therefore for me FIDO-U2F is only a nice to have now. (No real need to have.) You may close the request if you like.