R1:76d0b67d91cf

Remove "dateTouched" from ConpherenceParticipant

Summary:
Pathway to D17685. This column is (mostly) a denormalization of `dateModified` on the thread.

Just use a JOIN instead.

This isn't //exactly// the same: we'll bump threads to the top now for non-message changes (e.g., a topic or title change). That seems fine, but we could put a `lastMessageDate` on Thread later if we want to refine it.

Also got rid of a lot of other unused stuff. There's a big garbage TODO here, I'll fix that in the next change.