[server] Add silenceMessages to UpdateThreadOptions
[server] Add silenceMessages to UpdateThreadOptions

Summary: I want to be able to call `updateThread` without creating any new messages. Last time we ran a script that called `updateThread` it spammed all of our users...

Test Plan: Will be tested in combination with following diffs

Reviewers: atul

Reviewed By: atul

Subscribers: KatPo, palys-swm, Adrian

Differential Revision: https://phabricator.ashoat.com/D1242

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