Use Class and Property Decorators (#3730)

ref https://tc39.github.io/proposal-decorators/

app/javascript/mastodon/components/button.js

@@ -2,7 +2,7 @@ import React from 'react';
 import PropTypes from 'prop-types';
 import classNames from 'classnames';
 
-class Button extends React.PureComponent {
+export default class Button extends React.PureComponent {
 
   static propTypes = {
     text: PropTypes.node,
@@ -61,5 +61,3 @@ class Button extends React.PureComponent {
   }
 
 }
-
-export default Button;