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_templates.py

@@ -2169,7 +2169,7 @@ def test_should_show_page_for_a_deleted_template(
 def test_route_permissions(
     route,
     mocker,
-    app_,
+    notify_admin,
     client,
     api_user_active,
     service_one,
@@ -2181,7 +2181,7 @@ def test_route_permissions(
                  return_value='2012-01-01 12:00:00')
     validate_route_permission(
         mocker,
-        app_,
+        notify_admin,
         "GET",
         200,
         url_for(
@@ -2196,7 +2196,7 @@ def test_route_permissions(
 
 def test_route_permissions_for_choose_template(
     mocker,
-    app_,
+    notify_admin,
     client,
     api_user_active,
     mock_get_template_folders,
@@ -2207,7 +2207,7 @@ def test_route_permissions_for_choose_template(
     mocker.patch('app.job_api_client.get_job')
     validate_route_permission(
         mocker,
-        app_,
+        notify_admin,
         "GET",
         200,
         url_for(
@@ -2227,7 +2227,7 @@ def test_route_permissions_for_choose_template(
 def test_route_invalid_permissions(
     route,
     mocker,
-    app_,
+    notify_admin,
     client,
     api_user_active,
     service_one,
@@ -2236,7 +2236,7 @@ def test_route_invalid_permissions(
 ):
     validate_route_permission(
         mocker,
-        app_,
+        notify_admin,
         "GET",
         403,
         url_for(