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/test_utils.py

@@ -180,7 +180,7 @@ def test_spreadsheet_checks_for_bad_arguments(args, kwargs):
     ),
 ])
 def test_generate_notifications_csv_without_job(
-    app_,
+    notify_admin,
     mocker,
     created_by_name,
     expected_content,
@@ -224,7 +224,7 @@ def test_generate_notifications_csv_without_job(
     ),
 ])
 def test_generate_notifications_csv_returns_correct_csv_file(
-    app_,
+    notify_admin,
     mocker,
     _get_notifications_csv_mock,
     original_file_contents,
@@ -242,7 +242,7 @@ def test_generate_notifications_csv_returns_correct_csv_file(
 
 
 def test_generate_notifications_csv_only_calls_once_if_no_next_link(
-    app_,
+    notify_admin,
     _get_notifications_csv_mock,
 ):
     list(generate_notifications_csv(service_id='1234'))
@@ -252,7 +252,7 @@ def test_generate_notifications_csv_only_calls_once_if_no_next_link(
 
 @pytest.mark.parametrize("job_id", ["some", None])
 def test_generate_notifications_csv_calls_twice_if_next_link(
-    app_,
+    notify_admin,
     mocker,
     job_id,
 ):