Messagegroups are a new kind of group. Every user can join and leave such a
group on her own. They are useful to create topics for mails.
E.g. if there is a weekly newsletter we can create a new messagegroup for
it and every user which is interested in receiving the newsletter can join
the corresponding group.
This new view sorts the messages by threads based on the reply_to.
Now we store only the message which started a new thread in the reply_to
field to avoid recursive queries.
