[web] Move onSubmit up from ThreadSettingsModal to ConnectedThreadSettingsModal

Summary:
Move onSubmit out from inner class component to outer "Connected" functional component.

As part of the work to refactor ThreadSettingsModal... specifically to turn it into a functional component (any day now...)

Depends on D3795

Test Plan:

1. Open ThreadSettingsModal
2. Mess with the settings
3. Hit save and make sure things continue to look/work as expected

Reviewers: palys-swm, def-au1t

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3796