[web] Move deleteThreadAction up from ThreadSettingsModal to ConnectedThreadSettingsModal

Summary:
Move deleteThreadAction 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

Depends on D3770

Test Plan:

1. Open ThreadSettingsModal
2. Go to Delete tab
3. Enter password and delete thread
4. Ensure that things look/behave as expected

Reviewers: def-au1t, palys-swm

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3783