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

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

Doing this 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 D3821

Test Plan:

1. Open ThreadSettingsModal
2. Make sure the "main content" of the ThreadSettingsModal 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, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3822