[web] Push onChangeThreadType callback down to ThreadSettingsPrivacyTab component

Summary:
Push onChangeThreadType callback down to ThreadSettingsPrivacyTab component to clean up parent ThreadSettingsModal component and to move callback closer to where it's used.

• Added threadInfo, queuedChanges, and setQueuedChanges props to ThreadSettingsPrivacyTab component so callback could be "constructed" from "within" the child component
• Adding those props also lets us determine possiblyChangedThreadType within child component instead of having to pass in from parent

Depends on D3827

Test Plan:

1. Open ThreadSettingsModal
2. Make sure that the "Privacy" tab continues to look/work as expected

Reviewers: def-au1t, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3828