OTP in settings

Added functionality to add/remove YubiKeys and it's now possible to set if OTP is required when signing in.

Settings page: https://fantas.in/i/r0A6M.png
Configure OTP modal: https://fantas.in/i/qg2io.png
Add OTP modal: https://fantas.in/i/FWM7c.png

It's also probably best if the code is tested before it's published on the live server, so I did not forget to commit any files.