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/getting_started/index.js

@@ -30,6 +30,11 @@ const mapStateToProps = state => ({
 
 class GettingStarted extends ImmutablePureComponent {
 
+  static propTypes = {
+    intl: PropTypes.object.isRequired,
+    me: ImmutablePropTypes.map.isRequired
+  };
+
   render () {
     const { intl, me } = this.props;
 
@@ -66,9 +71,4 @@ class GettingStarted extends ImmutablePureComponent {
   }
 }
 
-GettingStarted.propTypes = {
-  intl: PropTypes.object.isRequired,
-  me: ImmutablePropTypes.map.isRequired
-};
-
 export default connect(mapStateToProps)(injectIntl(GettingStarted));