[web] Push changeThreadSettingsAction down to ThreadSettingsPrivacyTab

Summary:
Push down changeThreadSettingsAction (and accompanying state/functionality) to ThreadSettingsPrivacyTab.

Also push down the "Save" button.

In previous diff (D3842) did the same for ThreadSettingsGeneralTab... yes there's some repeated code but I think it's the best place to put it

Depends on D3842

Test Plan:

1. Open ThreadSettingsModal
2. Make some changes on the Privacy tab
3. Make sure that things continue 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/D3844