Add workaround fix for Webpack 4 & Premium plugin

We need this until https://github.com/webpack/webpack/pull/8614 is merged into Webpack.

[MAILPOET-1667]

views/layout.html

@@ -72,6 +72,7 @@ jQuery('.toplevel_page_mailpoet-newsletters.menu-top-last')
 <% block after_translations %><% endblock %>
 
 <%= javascript(
+  'admin_vendor_chunk.js',
   'admin_vendor.js'
 )%>
 

webpack.config.js

@@ -275,10 +275,22 @@ const adminConfig = {
           priority: 1,
           enforce: true,
         },
-        admin_vendor: {
-          name: 'admin_vendor',
+        admin_vendor_chunk: {
+          name: 'admin_vendor_chunk',
+          test: (module, chunks) => {
+            // add all modules from 'admin_vendor' entrypoint
+            if (chunks.some((chunk) => chunk.name === 'admin_vendor')) {
+              return true;
+            }
+
+            // add admin/form_editor/newsletter_editor shared modules
+            const filteredChunks = chunks.filter((chunk) => {
+              return ['admin', 'form_editor', 'newsletter_editor'].includes(chunk.name);
+            });
+            return filteredChunks.length > 1;
+          },
+          enforce: true,
           chunks: (chunk) => ['admin_vendor', 'admin', 'form_editor', 'newsletter_editor'].includes(chunk.name),
-          minChunks: 2,
           priority: 0,
         },
       }