Conpherence - implement PhabricatorDestructibleInterface so threads can be deleted
Summary: Fixes T7694. I had to complicate the ConpherenceThreadQuery code slightly so that if we specify id(s) or phid(s) then we don't bother with all that join stuff we need to make sure we have a reasonable query in production.
Test Plan: bin/remove destroy ZXX worked! tried to visit ZXX and got a nice 404. Clicked around and couldn't find anything broken because of the deletion
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D12304