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_service_settings.py

@@ -2071,7 +2071,7 @@ def test_ready_to_go_live(
 ])
 def test_route_permissions(
         mocker,
-        app_,
+        notify_admin,
         client,
         api_user_active,
         service_one,
@@ -2085,7 +2085,7 @@ def test_route_permissions(
 ):
     validate_route_permission(
         mocker,
-        app_,
+        notify_admin,
         "GET",
         200,
         url_for(route, service_id=service_one['id']),
@@ -2107,7 +2107,7 @@ def test_route_permissions(
 ])
 def test_route_invalid_permissions(
         mocker,
-        app_,
+        notify_admin,
         client,
         api_user_active,
         service_one,
@@ -2117,7 +2117,7 @@ def test_route_invalid_permissions(
 ):
     validate_route_permission(
         mocker,
-        app_,
+        notify_admin,
         "GET",
         403,
         url_for(route, service_id=service_one['id']),
@@ -2135,7 +2135,7 @@ def test_route_invalid_permissions(
 ])
 def test_route_for_platform_admin(
         mocker,
-        app_,
+        notify_admin,
         client,
         platform_admin_user,
         service_one,
@@ -2149,7 +2149,7 @@ def test_route_for_platform_admin(
 ):
     validate_route_permission(
         mocker,
-        app_,
+        notify_admin,
         "GET",
         200,
         url_for(route, service_id=service_one['id']),