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

Eventually we could add something like `%{ticket.thread.all}` or just `%{ticket.thread}` to represent the entire ticket thread so far; however, we need a way to indicate if internal notes should be included