#34 Forum/ML sync with threads

[Henning Hraban Ramm]

post-nuke.net team is working at "mailing list to forum"
functions. "forum to mailing list" is possible via
subscription.

At the moment every mail in ML2F starts a new thread.
I'd wish some threading via MsgID and/or Subject
recognition.

Together with configurable subscription mails (see my
other feature request) this would make a usable ML/F
sync.