Safer Conference::getContactId API.

Before getContactId would register a new contact using values
loaded from $_REQUEST. But it was called in contexts where
$_REQUEST had not been properly sanitized. This was actually OK
because Contact::load_by_email only used safe values from the
$_REQUEST object (e.g. voicePhoneNumber), but it still leaves a
bad taste. So use a separate registration object that contains
just the desired values.
Internal: Shift to Contact::id_by_email, load_by_email.
Remove Conference::getContactId in favor of Contact::load_by_email.

And restrict Contact::load_by_email API.