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.

templates/admin_newsletter_create.html

@@ -4,10 +4,9 @@
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>Create Newsletter</title>
+    <link rel="stylesheet" href="/static/css/styles.css" />
     <style>
       body {
-        font-family: Arial, sans-serif;
-        margin: 20px;
         max-width: 1200px;
         margin: 0 auto;
         padding: 20px;