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/status/components/card.js

@@ -19,6 +19,10 @@ const getHostname = url => {
 
 class Card extends React.PureComponent {
 
+  static propTypes = {
+    card: ImmutablePropTypes.map
+  };
+
   renderLink () {
     const { card } = this.props;
 
@@ -93,8 +97,4 @@ class Card extends React.PureComponent {
   }
 }
 
-Card.propTypes = {
-  card: ImmutablePropTypes.map
-};
-
 export default Card;