Rename "app_" fixture to "notify_admin"

This naming was introduced in 2016 without explanation [1]. I find it
confusing because:

- It's reminiscent of "_app", which is a Python convention indicating
the variable is internal, so maybe avoid using it.

- It suggests there's some other "app" fixture I should be using (there
isn't, though).

The Python style guide describes using an underscore suffix to avoid
clashes with inbuilt names [1], which is sort of applicable if we need
to import the "app" module [2]. However, we can also avoid clashes by
choosing a different name, without the strange underscore.

[1]: 3b1d521c10
[2]: 78824f54fd/tests/app/main/views/test_forgot_password.py (L5)

tests/app/main/views/test_tour.py

@@ -102,7 +102,7 @@ def test_should_404_if_no_mobile_number_for_tour_start(
 
 def test_should_403_if_user_does_not_have_send_permissions_for_tour_start(
     mocker,
-    app_,
+    notify_admin,
     client,
     api_user_active,
     mock_get_service_template_with_multiple_placeholders,
@@ -111,7 +111,7 @@ def test_should_403_if_user_does_not_have_send_permissions_for_tour_start(
 ):
     validate_route_permission(
         mocker,
-        app_,
+        notify_admin,
         "GET",
         403,
         url_for(
@@ -227,7 +227,7 @@ def test_should_404_for_get_tour_step_0(
 @pytest.mark.parametrize('method', ['GET', 'POST'])
 def test_should_403_if_user_does_not_have_send_permissions_for_tour_step(
     mocker,
-    app_,
+    notify_admin,
     client,
     api_user_active,
     mock_get_service_template_with_multiple_placeholders,
@@ -237,7 +237,7 @@ def test_should_403_if_user_does_not_have_send_permissions_for_tour_step(
 ):
     validate_route_permission(
         mocker,
-        app_,
+        notify_admin,
         method,
         403,
         url_for(