Support ticket thread items in email templates

* %{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
Merge pull request #790 from greezybacon/feature/thread-messages-in-template

Support ticket thread items in email templates

Reviewed-By: Peter Rotich <peter@osticket.com>