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/ui/components/columns_area.js

@@ -3,6 +3,10 @@ import PropTypes from 'prop-types';
 
 class ColumnsArea extends React.PureComponent {
 
+  static propTypes = {
+    children: PropTypes.node
+  };
+
   render () {
     return (
       <div className='columns-area'>
@@ -13,8 +17,4 @@ class ColumnsArea extends React.PureComponent {
 
 }
 
-ColumnsArea.propTypes = {
-  children: PropTypes.node
-};
-
 export default ColumnsArea;