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/notifications/components/notification.js

@@ -11,6 +11,10 @@ import ImmutablePureComponent from 'react-immutable-pure-component';
 
 class Notification extends ImmutablePureComponent {
 
+  static propTypes = {
+    notification: ImmutablePropTypes.map.isRequired
+  };
+
   renderFollow (account, link) {
     return (
       <div className='notification notification-follow'>
@@ -82,8 +86,4 @@ class Notification extends ImmutablePureComponent {
 
 }
 
-Notification.propTypes = {
-  notification: ImmutablePropTypes.map.isRequired
-};
-
 export default Notification;