Add page for security keys with stubbed data

This adds a new platform admin settings row, leading a page which
shows any existing keys and allows a new one to be registered. Until
the APIs for this are implemented, the user API client just returns
some stubbed data for manual testing.

This also includes a basic JavaScript module to do the main work of
registering a new authenticator, to be implemented in the next commits.

Some more minor notes:

- Setting the headings in the mapping_table is necessary to get the
horizontal rule along the top (to match the design).

- Setting caption to False in the mapping_table is necessary to stop
an extra margin appearing at the top.

tests/app/notify_client/test_user_client.py

@@ -238,3 +238,8 @@ def test_add_user_to_service_calls_correct_endpoint_and_deletes_keys_from_cache(
         call('service-{service_id}-template-folders'.format(service_id=service_id)),
         call('service-{service_id}'.format(service_id=service_id)),
     ]
+
+
+def test_get_webauthn_credentials_for_user_returns_stubbed_data():
+    credentials = user_api_client.get_webauthn_credentials_for_user('id')
+    assert credentials[0]['name'] == 'Ben test'