Label and use UNSAFE_componentWillMount to minimize warnings

These TODO comments are expected to be fixed ASAP, but until that happens let's minimize the errors in the console for development.

For https://github.com/vector-im/riot-web/issues/12877

These all aren't using componentDidMount because they do something which causes application instability if componentDidMount were used. Much of these calls are expected to move into constructors once they are converted to real classes.

src/components/views/messages/MessageEvent.js

@@ -47,6 +47,7 @@ export default createReactClass({
         maxImageHeight: PropTypes.number,
     },
 
+    // TODO: [REACT-WARNING] Replace component with real class, use constructor for refs
     UNSAFE_componentWillMount: function() {
         this._body = createRef();
     },