MDL-48164 mod_forum: Add additional headers to prevent OOF replies

This adds the X-Auto-Response-Suppress header to outbound messages.
MDL-48164 messageinbound: Attempt to detect autoreplies

Any message which includes:
* Precedence: bulk
* X-Autoreply: !no
* X-Autoresponse: !no

Will not be detected as a bulk message and therefore ignored.