Fix WebAuthn registration to use random bytes for user handle
Fix WebAuthn registration to use random bytes for user handle

User handle is used in a username-less authentication, to map a
credential received from browser with an user account in the
database. Since we only use security keys as a second factor,
the user handle is not of much use to us.

The user handle:
 - must not be blank,
 - must not be a constant value,
 - must not contain personally identifiable information.

So we use random bytes, and don't store them on our end.

healthchecks/healthchecks,iphoting/healthchecks,healthchecks/healthchecks,iphoting/healthchecks,healthchecks/healthchecks,iphoting/healthchecks,healthchecks/healthchecks,iphoting/healthchecks