Remove requirement of external-ticket-id in subject line

This patch affords an administrator the ability to remove the [#%{ticket.number}] from the email template subject line for the new ticket autoresponse and the new message autoresponse. Previously, the ticket number with a prefixed hash in brackets was used to identify which ticket thread an email was in reference to.

With this patch, the email message-id (which was already kept on file) is sent in the MIME "References" header. When a user responds to an autoresponse email, the "References" will include this message-id in the return email. The ticket thread is then matched up with the email based on the message-id rather than the subject line.

Ticket numbers are still supported in the subject line, in the event that non-compliant email clients do not properly include the References header.