Add support for a 2-factor auth system such as U2F

[mightybyte]

It would really be nice to be able to use some kind of 2FA (such as a yubikey) to improve my clipperz security.

[ghost]

totally agree! one password is definitely to weak.

I worked once on a foreign pc which had a virus! That started sending spam mails in my name. Now i use google two factor authentication and change my password each quarter.

[EdyTheCow]

I would love to see yubikey support! Clipperz not having a support for some sort of 2-factor is the only reason keeping me from moving to Clipperz.

[scippio]

There is javascript library https://github.com/ashtuchkin/u2f