Don’t let people actually start a letter job

Who knows what would happen if a job with a letter template actually
got into the database. `403`ing the page is a quick and dirty hack to
stop this from happening.

tests/conftest.py

@@ -335,6 +335,21 @@ def mock_get_service_email_template(mocker):
         'app.service_api_client.get_service_template', side_effect=_create)
 
 
+@pytest.fixture(scope='function')
+def mock_get_service_letter_template(mocker):
+    def _create(service_id, template_id):
+        template = template_json(
+            service_id,
+            template_id,
+            "Two week reminder",
+            "letter",
+            "Your vehicle tax is about to expire", "Subject")
+        return {'data': template}
+
+    return mocker.patch(
+        'app.service_api_client.get_service_template', side_effect=_create)
+
+
 @pytest.fixture(scope='function')
 def mock_create_service_template(mocker, fake_uuid):
     def _create(name, type_, content, service, subject=None):