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