Mark client fixture as ‘private’

No tests are now using the `client` fixture directly so we can rename
it.

Python convention is to use an `_underscore` for things which should be
considered semi private.

This should discourage people from writing new tests with these old
fixtures.

New tests should always use `client_request`.

Want to be logged in with a different user? Call
`client_request.login(user)` first.

Don’t want to be logged in? Call `client_request.logout()` first (most
of our tests need to be logged in).

Need an instance of `Response` object not an instance of
`BeautifulSoup`? Use `client_request.get_response` or
`client_request.post_response`.

Need to pass in a URL, not arguments to `url_for`? Use
`client_request.get_url(…)` or `client_request.post_url(…)`.

Need to pass in a URL and get a response back? Use
`client_request.get_response_from_url(…)` or
`client_request.post_response_from_url(…)`.

tests/conftest.py

@@ -2732,21 +2732,27 @@ def mock_send_notification(mocker, fake_uuid):
 
 
 @pytest.fixture(scope='function')
-def client(notify_admin):
+def _client(notify_admin):
+    """
+    Do not use this fixture directly – use `client_request` instead
+    """
     with notify_admin.test_request_context(), notify_admin.test_client() as client:
         yield client
 
 
 @pytest.fixture(scope='function')
 def _logged_in_client(
-    client,
+    _client,
     active_user_with_permissions,
     mocker,
     service_one,
     mock_login
 ):
-    client.login(active_user_with_permissions, mocker, service_one)
-    yield client
+    """
+    Do not use this fixture directly – use `client_request` instead
+    """
+    _client.login(active_user_with_permissions, mocker, service_one)
+    yield _client
 
 
 @pytest.fixture