Update babel to 7.5, and actually enable polyfills

@babel/polyfill is deprecated in ≥ 7.4 [1], and we never actually used
it as we forgot to import it [2]. Move to core-js/stable.

[1] https://babeljs.io/blog/2019/03/19/7.4.0#migration-from-core-js-2
[2] https://babeljs.io/docs/en/babel-polyfill#usage-in-node-browserify-webpack

src/index.js

@@ -17,6 +17,8 @@
  * along with Cockpit; If not, see <http://www.gnu.org/licenses/>.
  */
 
+import "core-js/stable";
+
 import React from 'react';
 import ReactDOM from 'react-dom';
 import { Application } from './app.jsx';