This diff completely removes the remnants of the `ThreadSettingsModal` class component. Specifically, this diff lifts the `render()` function of the class component to the `return` value of the outer functional component.
Now that we've turned `ThreadSettingsModal` into a functional component, we're going to continue the refactor by pushing "tab-specific" state/functionality to the `ThreadSettings[General/Privacy/Delete]Tab` child components (which are also now functional components)
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Depends on D3824