make letters error if user not logged in

also added tests for the various hiding logic points

also added new logged_in_client in conftest - so you dont need to
patch all those stupid API calls for get user and get service

tests/app/main/views/test_letters.py

@@ -0,0 +1,17 @@
+import pytest
+from flask import url_for
+
+from tests import service_json
+
+
+@pytest.mark.parametrize('can_send_letters, response_code', [
+    (True, 200),
+    (False, 403)
+])
+def test_letters_access_restricted(logged_in_client, mocker, can_send_letters, response_code):
+    service = service_json(can_send_letters=can_send_letters)
+    mocker.patch('app.service_api_client.get_service', return_value={"data": service})
+
+    response = logged_in_client.get(url_for('main.letters', service_id=service['id']))
+
+    assert response.status_code == response_code