Place the message-id in the body of the email

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.
Predictable message-ids

Outbound message-ids are predictable, with a consistent code unique and
static to each osTicket installation. This will help detect email loops
where message is delivered back to the system in an email loop.
Log errors on email loop detection

If an email loop is detected, in addition to dropping the message, also log
an error to the system log, but do not send an email to the administrator in
the event that it is the administrator's email that is looped.
Merge pull request #584 from greezybacon/issue/outbound-message-ids

Place the message-id in the body of the email

Reviewed-By: Peter Rotich <peter@osticket.com>