Place the message-id in the body of the email

Add a message-id token to the body of the message. For HTML messages, it's put into a `data-mid` attribute and will not be shown by any mail client. For text messages, an `X-Mid: ` line is (*discretely*) added to the bottom of the body. The thread class will also now inspect the body for a message-id token in the body which connects directly to a thread entry.

This approach will help detect responses to the ticket thread from mail clients which do not include the References and In-Reply-To MIME headers. This also allows the Subject line to continue to omit the ticket number.