feat(newsletter): Add subscription confirmation email functionality

- Implemented `send_subscription_confirmation` function to send a confirmation email upon subscription.
- Added a default HTML template for the confirmation email in settings.
- Updated the newsletter management page to handle subscription and unsubscription actions.
- Created new templates for embedding contact and newsletter forms.
- Added styles for the new templates and unified CSS styles across the application.
- Updated tests to reflect changes in the newsletter management API endpoints.

iframe_contact.html

@@ -0,0 +1 @@
+<iframe src="http://your-server-domain/embed/contact" width="600" height="400" frameborder="0" allowfullscreen></iframe>