Use ES Class Fields & Static Properties (#3008)

Use ES Class Fields & Static Properties (currently stage 2) for improve class outlook.

Added babel-plugin-transform-class-properties as a Babel plugin.

app/javascript/mastodon/features/compose/components/warning.js

@@ -3,9 +3,9 @@ import PropTypes from 'prop-types';
 
 class Warning extends React.PureComponent {
 
-  constructor (props) {
-    super(props);
-  }
+  static propTypes = {
+    message: PropTypes.node.isRequired
+  };
 
   render () {
     const { message } = this.props;
@@ -19,8 +19,4 @@ class Warning extends React.PureComponent {
 
 }
 
-Warning.propTypes = {
-  message: PropTypes.node.isRequired
-};
-
 export default Warning;