Use the new JSX transform (#25064)

app/javascript/mastodon/components/column_header.jsx

@@ -1,4 +1,4 @@
-import React from 'react';
+import { PureComponent } from 'react';
 import PropTypes from 'prop-types';
 import { createPortal } from 'react-dom';
 import classNames from 'classnames';
@@ -12,7 +12,7 @@ const messages = defineMessages({
   moveRight: { id: 'column_header.moveRight_settings', defaultMessage: 'Move column to the right' },
 });
 
-class ColumnHeader extends React.PureComponent {
+class ColumnHeader extends PureComponent {
 
   static contextTypes = {
     router: PropTypes.object,