Paginate search results for notifications

The standard way that we indicate that there are more results than can
be returned is by paginating. So even though we don’t intend to paginate
the search results in the admin app, it can still use the presence or
absence of a ‘next’ link to determine whether or not to show a message
about only showing the first 50 results.

tests/app/service/test_rest.py

@@ -2103,6 +2103,24 @@ def test_search_for_notification_by_to_field_return_multiple_matches(client, sam
     assert str(notification4.id) not in notification_ids
 
 
+def test_search_for_notification_by_to_field_returns_next_link_if_more_than_50(
+    client, sample_template
+):
+    for i in range(51):
+        create_notification(sample_template, to_field='+447700900855', normalised_to='447700900855')
+
+    response = client.get(
+        '/service/{}/notifications?to={}&template_type={}'.format(sample_template.service_id, '+447700900855', 'sms'),
+        headers=[create_authorization_header()]
+    )
+    assert response.status_code == 200
+    response_json = json.loads(response.get_data(as_text=True))
+
+    assert len(response_json['notifications']) == 50
+    assert 'prev' not in response_json['links']
+    assert 'page=2' in response_json['links']['next']
+
+
 def test_search_for_notification_by_to_field_for_letter(
     client,
     notify_db,