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.

tests/test_newsletter_api.py

@@ -76,13 +76,13 @@ def test_newsletter_update_email_not_found(client):
 
 
 def test_newsletter_manage_page_get(client):
-    resp = client.get("/api/newsletter/manage")
+    resp = client.get("/newsletter/manage")
     assert resp.status_code == 200
     assert b"Newsletter Subscription Management" in resp.data
 
 
 def test_newsletter_manage_subscribe(client):
-    resp = client.post("/api/newsletter/manage",
+    resp = client.post("/newsletter/manage",
                        data={"email": "manage@example.com", "action": "subscribe"})
     assert resp.status_code == 200
     assert b"Successfully subscribed" in resp.data
@@ -90,29 +90,29 @@ def test_newsletter_manage_subscribe(client):
 
 def test_newsletter_manage_unsubscribe(client):
     # Subscribe first
-    client.post("/api/newsletter/manage",
+    client.post("/newsletter/manage",
                 data={"email": "manage@example.com", "action": "subscribe"})
     # Unsubscribe
-    resp = client.post("/api/newsletter/manage",
+    resp = client.post("/newsletter/manage",
                        data={"email": "manage@example.com", "action": "unsubscribe"})
     assert resp.status_code == 200
-    assert b"Successfully unsubscribed" in resp.data
+    assert b"Unsubscription Confirmed" in resp.data
 
 
 def test_newsletter_manage_update(client):
     # Subscribe first
-    client.post("/api/newsletter/manage",
+    client.post("/newsletter/manage",
                 data={"email": "old@example.com", "action": "subscribe"})
     # Update
-    resp = client.post("/api/newsletter/manage", data={
-                       "old_email": "old@example.com", "new_email": "updated@example.com", "action": "update"})
+    resp = client.post("/newsletter/manage", data={
+                       "old_email": "old@example.com", "email": "updated@example.com", "action": "update"})
     assert resp.status_code == 200
     # Check that some success message is displayed
-    assert b"success" in resp.data.lower() or b"updated" in resp.data.lower()
+    assert b"updated successfully" in resp.data.lower()
 
 
 def test_newsletter_manage_invalid_email(client):
-    resp = client.post("/api/newsletter/manage",
+    resp = client.post("/newsletter/manage",
                        data={"email": "invalid-email", "action": "subscribe"})
     assert resp.status_code == 200
     assert b"Please enter a valid email address" in resp.data