Deleting forum posts should update 'last reply' info

[gasman]

We store the time and user of the most recent reply as part of the Topic record, so we can show these on the listing on the homepage (and use them for sorting that listing). Deleting the most recent reply should cause these to be updated to the next most recent one - currently it's left unchanged, so if an old topic is bumped by a spambot it remains at the top even after the spam is deleted.