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/models/test_service.py

@@ -74,7 +74,7 @@ def _get_all_folders(active_user_with_permissions):
 
 
 def test_get_user_template_folders_only_returns_folders_visible_to_user(
-    app_,
+    notify_admin,
     mock_get_template_folders,
     service_one,
     active_user_with_permissions,
@@ -118,7 +118,7 @@ def test_get_user_template_folders_only_returns_folders_visible_to_user(
 
 
 def test_get_template_folders_shows_user_folders_when_user_id_passed_in(
-    app_,
+    notify_admin,
     mock_get_template_folders,
     service_one,
     active_user_with_permissions,