()BUG) Improve the updating of Topics after reply deletion

I notice that If replies are added and then deleted, the TOPIC doesn't get updated in its "last_reply" and "Modified_by" fields!

These should be updated when an asmin comes along and unpublishes/trashes/permanently deletes a reply from the thread.

Otherwise the "recent topics" page ( ?view=topics&recent=true ) will show the wrong person replying and a link to nonexisting reply...