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/test_request_header.py

@@ -9,14 +9,14 @@ from tests.conftest import set_config_values
     (False, 'wrong_key', 200),
     (False, 'key_1', 200),
 ])
-def test_route_correct_secret_key(app_, check_proxy_header, header_value, expected_code):
-    with set_config_values(app_, {
+def test_route_correct_secret_key(notify_admin, check_proxy_header, header_value, expected_code):
+    with set_config_values(notify_admin, {
         'ROUTE_SECRET_KEY_1': 'key_1',
         'ROUTE_SECRET_KEY_2': '',
         'CHECK_PROXY_HEADER': check_proxy_header,
     }):
 
-        with app_.test_client() as client:
+        with notify_admin.test_client() as client:
             response = client.get(
                 path='/_status?elb=True',
                 headers=[