R1:d24b3dcb7de3

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

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