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