Use commons chunk for JS code shared between entry points

This is a replacement for previous admin_vendor_chunk
see 422245e3e6/webpack.config.js (L235-L251)
The automatic splitting based on minChunks seems to give better results.
[MAILPOET-3214]

webpack.config.js

@@ -298,7 +298,11 @@ const adminConfig = {
     runtimeChunk: 'single',
     splitChunks: {
       cacheGroups: {
-        chunks: 'all',
+        commons: {
+          name: 'commons',
+          chunks: 'initial',
+          minChunks: 2,
+        },
       },
     }
   },