Add a way to view your 2FA backup codes
Fixes #213 (closed).
This adds another form to the 2FA settings (if enabled) that lets you view your backup codes.
There's also a separate commit that adds generating new codes to the viewing page. I'm not sure if this is safe yet because I think you could just POST to that API endpoint and generate new codes without having entered a code before, I don't know how to do that so I'll leave it up to you (apart from adding a third form to the 2FA settings that's just for generating new codes). :P