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_webauthn_credential.py

@@ -47,7 +47,7 @@ def test_from_registration_encodes_as_unicode(webauthn_dev_server):
     assert type(serialized_credential['registration_response']) == str
 
 
-def test_from_registration_handles_library_errors(app_):
+def test_from_registration_handles_library_errors():
     registration_response = {
        'clientDataJSON': CLIENT_DATA_JSON,
        'attestationObject': ATTESTATION_OBJECT,