[web] Lift buttons: React.Node from ThreadSettingsModal to ConnectedThreadSettingsModal

Summary:
Lift button from the ThreadSettingsModal class component to the ConnectedThreadSettingsModal functional component.

Doing this to "stage" the lifting of JSX markup into separate steps to hopefully make it easier to review.

As part of the work to refactor ThreadSettingsModal... specifically to turn it into a functional component.

Depends on D3822

Test Plan:

1. Open ThreadSettingsModal
2. Make sure the "buttons" of the ThreadSettingsModal component continues to look and behave as expected (since that was what was moved)
3. Change some settings around, hit save, make sure things are saved as expected

Reviewers: def-au1t, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3823