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