[web] Move namePlaceholder up from ThreadSettingsModal to ConnectedThreadSettingsModal

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

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

Depends on D3760

Test Plan:

1. Opened ThreadSettingsModal for a thread
2. Ensured that the placeholder was as expected

Reviewers: def-au1t, palys-swm

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3762