Make failed logins for MFA-enabled accounts return a consistent response
and timing regardless of the cause, in line with authentication best
practices. Guidance for CLI users whose client can't complete email
verification is now surfaced by fleetctl on any login failure.
Added a `user_mfa_requested` activity, recorded when valid credentials
are submitted for an MFA-enabled account and a verification email is
sent.
Juan Fernandez
e91a0b2987