Forbid message loops for alert messages

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.
Merge pull request #714 from greezybacon/issue/thread-alert-loop

Forbid message loops for alert messages

Reviewed-By: Peter Rotich <peter@osticket.com>