[server] Update community in updateThread
[server] Update community in updateThread

Summary: Now that we can make sure that `parentThreadID` changes don't change the containing community, we can reenable them (despite it not being done anywhere in the UI).

Test Plan: Will be tested in combination with following diffs

Reviewers: atul

Subscribers: KatPo, palys-swm, Adrian

Differential Revision: https://phabricator.ashoat.com/D1309

Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal