Upgrade webpack 2 (#4748)

* update webpack to version 4

* repair build bug

* update package code

* optimize package script

* update seafile-editor version

* repair code bug

frontend/config/polyfills.js

@@ -1,25 +0,0 @@
-'use strict';
-
-require('react-app-polyfill/ie9');
-require('react-app-polyfill/stable');
-
-if (typeof Promise === 'undefined') {
-  // Rejection tracking prevents a common issue where React gets into an
-  // inconsistent state due to an error, but it gets swallowed by a Promise,
-  // and the user has no idea what causes React's erratic future behavior.
-  require('promise/lib/rejection-tracking').enable();
-  window.Promise = require('promise/lib/es6-extensions.js');
-}
-
-// fetch() polyfill for making API calls.
-require('whatwg-fetch');
-
-// Object.assign() is commonly used with React.
-// It will use the native implementation if it's present and isn't buggy.
-Object.assign = require('object-assign');
-
-// In tests, polyfill requestAnimationFrame since jsdom doesn't provide it yet.
-// We don't polyfill it in the browser--this is user's responsibility.
-if (process.env.NODE_ENV === 'test') {
-  require('raf').polyfill(global);
-}