feat: Link help center portal to an Inbox (#6903)

app/views/api/v1/models/_inbox.json.jbuilder

@@ -17,6 +17,13 @@ json.callback_webhook_url resource.callback_webhook_url
 json.allow_messages_after_resolved resource.allow_messages_after_resolved
 json.lock_to_single_conversation resource.lock_to_single_conversation
 
+if resource.portal.present?
+  json.help_center do
+    json.name resource.portal.name
+    json.slug resource.portal.slug
+  end
+end
+
 ## Channel specific settings
 ## TODO : Clean up and move the attributes into channel sub section