[web] Fix canSeePrivacyTab logic in ThreadSettingsModal

Summary:
Logic was originally introduced to hide privacy tab for subthreads/subchannels that were "promoted" from sidebars: https://github.com/CommE2E/comm/commit/57588fae748567e0d73e5a85dd2eb536d14e8caf

Context for that logic is here: https://phabricator.ashoat.com/D1000?id=3007#inline-5622

Diff where threadInfo.sourceMessageID condition was flipped is here: https://github.com/CommE2E/comm/commit/0cfdd365eed90bef77bcc630a7076ce73d5ec2ad

Test Plan: Able to see the "Privacy" tab on subthreads/subchannels that were created directly... not able to see the "Privacy" tab on threads that were originally sidebars

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3938