Previously, the References header emitted by the system for new thread
entry auto-responses and alerts included the message-id for which the email
was a reply in the `References` header. This patch corrects the issue by
placing the reply message-id in the In-Reply-To header, and constructs a
correct References header.

That is, if available, the References header received in the email that was
used to create the thread entry is appended to the message-id the email is
actually in reply to. This is the expected usage of the References header.

* %{ticket.thread.original} represents the original message submitted when
    creating the ticket
* %{ticket.thread.lastmessage} represents the most recent message submitted
    by the end user

line is used.

id/number.

* Ticket number will be customization in the future
* Ticket thread only link to Ticket should be internal ticket id.

if the References or In-Reply-To header matches in a way that continues a
ticket's thread, include the attachments in the thread if allowed by the
system settings.

In the event that the system is set to do-nothing for POP fetched
emails, and eventually, one of the emails is rejected, ensure that the
message-id is logged so that the message will not be re-fetched, even if it
is undeleted.

When comparing the From address of incoming email. If the ticket owner sent
an email back into the system and the email address did not match exactly,
case-wise, the email would not be considered from the ticket owner.

If an alert message manages to loop back into the ticketing system, refuse
posting to the ticket thread. Technically, the message should be marked as
an auto-response message; however, auto-response messages should usually be
allowed to be appended to the ticket thread.

This patch will check if the From email header cites an email address that
is a system email address (visible in the Emails section of the Admin
Panel). If it is, the email is completely ignored.

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 and
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.

Fully implement thread entry class and types (Note, Message & Response)