WebAuthN support

The current U2F implementation will eventually become obsolete, we need to support WebAuthN/FIDO/etc. Unfortunately this requires finding another library to deal with the primitives other than github.com/tstranex/u2f, and it will have to support both U2F and WebAuthN.

This issue tracks implementation across all the components of the stack:

• ai3/go-common for the U2FRegistration type (ai3/go-common!32 (merged))
• id/auth for the server-side login workflow (!34 (merged))
• id/sso-server for the client-side login workflow (sso-server!8 (merged))
• ai3/pannello for the registration workflow (ai3/pannello!51)
• ai3/accountserver (ai3/accountserver!42 (merged))