Update notificaiton API endpoints to use template's reply_to

Sets the reply_to_text on notification from the template value if
it's set.

tests/app/v2/notifications/test_post_notifications.py

@@ -28,6 +28,7 @@ from tests.app.db import (
     create_service,
     create_template,
     create_reply_to_email,
+    create_letter_contact,
     create_service_sms_sender,
     create_service_with_inbound_number
 )
@@ -639,3 +640,26 @@ def test_post_email_notification_with_invalid_reply_to_id_returns_400(client, sa
     assert 'email_reply_to_id {} does not exist in database for service id {}'. \
         format(fake_uuid, sample_email_template.service_id) in resp_json['errors'][0]['message']
     assert 'BadRequestError' in resp_json['errors'][0]['error']
+
+
+def test_letter_notification_should_use_template_reply_to(client, sample_letter_template):
+    letter_contact = create_letter_contact(sample_letter_template.service, "Edinburgh, ED1 1AA", is_default=False)
+    sample_letter_template.reply_to = str(letter_contact.id)
+
+    data = {
+        'template_id': str(sample_letter_template.id),
+        'personalisation': {
+            'address_line_1': '123',
+            'address_line_2': '234',
+            'postcode': 'W1A1AA',
+        }
+    }
+    response = client.post("v2/notifications/letter",
+                           data=json.dumps(data),
+                           headers=[('Content-Type', 'application/json'),
+                                    create_authorization_header(service_id=sample_letter_template.service.id)]
+                           )
+    assert response.status_code == 201, response.get_data(as_text=True)
+    notifications = Notification.query.all()
+    assert len(notifications) == 1
+    assert notifications[0].reply_to_text == "Edinburgh, ED1 1AA"