Include sourceMessageID in all ThreadInfo structs

Summary: I actually don't think this should cause any issues with older clients, and since we don't have sidebars on prod yet, I think we can avoid having to run a script to update ThreadStores. Should be pretty easy!

Test Plan: Will test this together with the next diff, which will make the sourceMessageID field required when creating new sidebars

Reviewers: palys-swm, KatPo

Subscribers: zrebcu411, Adrian, atul, subnub

Differential Revision: https://phabricator.ashoat.com/D593