Previously, it was possible to have "Require sign-in" unchecked and a
"Security = None" setting for the outgoing server and still not be able to
tap "Next" because of a hidden (and irrelevant) "Authentication = Client
certificate" setting.
Check that the user has actually chosen a client certificate in
AccountSetupOutgoing.validateFields().
Also, there's no need to clear the password and certificate fields when
hiding them. The user may accidentally change settings and want to change
them back without wiping out the existing settings.