Clear PHPMailer Message-ID between messages

A single PHPMailer instance is reused for all messages sent within a
process, but reset() did not clear the MessageID property. Since only
notifications for newly submitted issues set a Message-ID, every
subsequent message inherited the value, so notifications for unrelated
issues were sent under an identical Message-ID.

An empty MessageID is the PHPMailer default and makes it generate a
unique one per message. Threading is unaffected, In-Reply-To is added as
a custom header and already cleared.

Fixes 0037355