Use personalisation to set client_reference for letters

which were sent through Notify interface only. This is done
to avoid performance dip from additional operation for
other notification types.

tests/app/notifications/rest/test_send_notification.py

@@ -1249,3 +1249,27 @@ def test_send_notification_should_send_international_letters(
     notification = Notification.query.get(notification_id['id'])
     assert notification.postage == expected_postage
     assert notification.international == expected_international
+
+
+@pytest.mark.parametrize('reference_paceholder,', [None, 'ref2'])
+def test_send_notification_should_set_client_reference_from_placeholder(
+    sample_letter_template, mocker, reference_paceholder
+):
+    deliver_mock = mocker.patch('app.celery.tasks.letters_pdf_tasks.get_pdf_for_templated_letter.apply_async')
+    data = {
+        'template_id': sample_letter_template.id,
+        'personalisation': {
+            'address_line_1': 'Jane',
+            'address_line_2': 'Moss Lane',
+            'address_line_3': 'SW1A 1AA',
+        },
+        'to': 'Jane',
+        'created_by': sample_letter_template.service.created_by_id
+    }
+    if reference_paceholder:
+        data['personalisation']['reference'] = reference_paceholder
+
+    notification_id = send_one_off_notification(sample_letter_template.service_id, data)
+    assert deliver_mock.called
+    notification = Notification.query.get(notification_id['id'])
+    assert notification.client_reference == reference_paceholder