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]

views/layout.html

@@ -77,6 +77,7 @@ jQuery('#adminmenu #toplevel_page_mailpoet-newsletters')
 <%= javascript(
   'runtime.js',
   'vendor.js',
+  'commons.js',
   'mailpoet.js'
 )%>
 

webpack.config.js

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