[web] Turn ThreadSettingsModal into functional component

Summary:
This diff completely removes the remnants of the ThreadSettingsModal class component. Specifically, this diff lifts the render() function of the class component to the return value of the outer functional component.

Now that we've turned ThreadSettingsModal into a functional component, we're going to continue the refactor by pushing "tab-specific" state/functionality to the ThreadSettings[General/Privacy/Delete]Tab child components (which are also now functional components)

Depends on D3822

Test Plan:

1. Open ThreadSettingsModal
2. Click around, make some changes, hit save
3. Make sure that everything continues to look/behave as expected

Reviewers: def-au1t, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3825