Fix VOIP bar hidden on first render of RoomStatusBar
componentDidUpdate is not called for the initial render of a React component (https://facebook.github.io/react/docs/react-component.html#componentdidupdate)
componentWillMount is used so that the initial state and props of RoomStatusBar can also trigger props.isVisible.
This fixes https://github.com/vector-im/riot-web/issues/3181
