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/navigation_bar.js

@@ -10,6 +10,10 @@ import ImmutablePureComponent from 'react-immutable-pure-component';
 
 class NavigationBar extends ImmutablePureComponent {
 
+  static propTypes = {
+    account: ImmutablePropTypes.map.isRequired
+  };
+
   render () {
     return (
       <div className='navigation-bar'>
@@ -30,8 +34,4 @@ class NavigationBar extends ImmutablePureComponent {
 
 }
 
-NavigationBar.propTypes = {
-  account: ImmutablePropTypes.map.isRequired
-};
-
 export default NavigationBar;