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
Maniphest Tasks: T7756, T7694
Differential Revision: https://secure.phabricator.com/D12304