[web] Don't show Privacy tab in ThreadSettingsModal for sidebars

Summary: More accurately: for threads created as sidebars. More context here.

Test Plan: Check ThreadSettingsModal for a thread created as a sidebar

Reviewers: KatPo, palys-swm

Reviewed By: KatPo, palys-swm

Subscribers: Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D1007