[web] Introduce minimally viable ThreadSettingsPrivacyTab component

Summary:
In an effort to make ThreadSettingsModal easier to work with, we're pulling general, privacy, and delete tabs out into their own separate components.

This diff does that for ThreadSettingsPrivacyTab, but in a limited way. We're basically moving over the mainContent JSX markup and prop drilling that state that's still held by ThreadSettingsModal.

Once we've extracted the JSX markup for general, privacy, and delete.. and prop drilled as necessary to keep things working as they did before: we're going to push down the state and functionality into the smaller "tab" components so things are properly encapsulated.

And once that's done, we should easily be able to turn ThreadSettingsModal into a functional component.. and refactor ThreadSettingsModal using @def-au1t's TabsContainer and TabsItem components.

Depends on D3716

Test Plan:
This diff should be a noop. I spend some time changing the thread privacy settings--and things continue to look and work as expected (we'll likely need a restyle of this tab, but that's for another diff):

Reviewers: def-au1t, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga, benschac, yayabosh, def-au1t

Differential Revision: https://phabricator.ashoat.com/D3717