Fix images in new ticket notice to end user

Images placed in the "Issue Details" in a new ticket by staff are not
correctly translated to the standard used in the storage model in the
%ticket_thread table nor for the standard used by the email system.

This patch changes the logic used to create the email template to fetch the
sanitized and converted message body from the ThreadEntry instance created
rather than reusing the raw message originally sent in the request.

References:
https://github.com/osTicket/osTicket-1.8/issues/816