Move onChangeDescription 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 D3765

1. Open ThreadSettingsModal
2. Change the thread description a bunch
3. Log values in onChangeDescription to ensure that they continue to be as expected
4. Hit save after changing the description and ensure that the thread description changed as expected