R1:080fb1985f29

Upgrade an old "weakDigest()" inside TOTP synchronization code

Summary:
Ref T13222. Ref T12509. When you add a new MFA TOTP authenticator, we generate a temporary token to make sure you're actually adding the key we generated and not picking your own key.

That is, if we just put inputs in the form like `key=123, response=456`, users could pick their own keys by changing the value of `key` and then generating the correct `response`. That's probably fine, but maybe attackers could somehow force users to…