Enforce loading babel-polyfill first

This is to avoid loading anything before loading the polyfill, otherwise
we risk loading dependencies (such as React) before it.

This was causing the compatibility page to fail somehow.

See https://github.com/facebook/react/issues/8379

Fixes https://github.com/vector-im/riot-web/issues/6562

webpack.config.js

@@ -8,6 +8,11 @@ if (!og_image_url) og_image_url = 'https://riot.im/app/themes/riot/img/logos/rio
 
 module.exports = {
     entry: {
+        main: [
+            // Load this first to avoid issues where some imports (namely react)
+            // potentially before babel-polyfill.
+            'babel-polyfill',
+        ],
         "bundle": "./src/vector/index.js",
         "indexeddb-worker": "./src/vector/indexeddb-worker.js",