Fix newsletter related broken eslint rules

assets/js/src/help/help.jsx

@@ -2,6 +2,7 @@ import React from 'react';
 import ReactDOM from 'react-dom';
 import { Router, Route, IndexRedirect, useRouterHistory } from 'react-router';
 import { createHashHistory } from 'history';
+import PropTypes from 'prop-types';
 
 import SystemStatus from 'help/system_status.jsx';
 import SystemInfo from 'help/system_info.jsx';
@@ -15,6 +16,10 @@ class App extends React.Component {
   }
 }
 
+App.propTypes = {
+  children: PropTypes.element.isRequired,
+};
+
 const container = document.getElementById('help_container');
 
 if (container) {