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.

server/routes/admin.py

@@ -82,6 +82,13 @@ def submissions():
     return render_template("admin_submissions.html")
 
 
+@bp.route("/embeds")
+@auth.login_required
+def embeds():
+    """Display embeddable forms management page."""
+    return render_template("admin_embeds.html")
+
+
 @bp.route("/api/settings", methods=["GET"])
 @auth.login_required
 def get_settings_api():