[web] Push "Delete thread" functionality down to ThreadSettingsDeleteTab

Summary:
Push "Delete thread" functionality (onDelete & deleteThreadAction) down to ThreadSettingsDeleteTab component.

Also made necessary "supporting changes":

• Move "Delete" button to ThreadSettingsDeleteTab
• Pass additional props to ThreadSettingsDeleteTab so deleteThreadAction and onDelete could be "recreated" from within the child component

(This component is definitely in need of restyling, but will sequence that a bit later in ThreadSettingsModal work)

Depends on D3832

Test Plan:

1. Open ThreadSettingsModal
2. Navigate to the "Delete" tab
3. Make sure that everything continues to look as expected

Reviewers: def-au1t, palys-swm

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3837