cockpit: Stop importing cockpit's base1/patternfly.css

This is deprecated API and will be dropped at some point, in favor
of projects shipping their own CSS.

Follow https://github.com/cockpit-project/starter-kit/blob/master/webpack.config.js
approach on how to use PF CSS.

Also:
* Replace extract-text-webpack-plugin with mini-css-webpack-plugin as
the former is deprecated: https://webpack.js.org/plugins/extract-text-webpack-plugin/#usage
* Upgrade PF module dependency
* Reduce the css files that are linked through webpack. Instead include
from the JS the CSS files and let mini-css-extract-plugin do the rest of
the work

src/index.es6

@@ -19,6 +19,9 @@
 
 import React from 'react';
 import ReactDOM from 'react-dom';
+
+import '../lib/patternfly/patternfly-cockpit.scss';
+
 import { Application } from './app.jsx';
 
 document.addEventListener("DOMContentLoaded", function () {