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.

app/notify_client/user_api_client.py

@@ -191,5 +191,13 @@ class UserApiClient(NotifyAdminAPIClient):
         endpoint = '/user/{}/organisations-and-services'.format(user_id)
         return self.get(endpoint)
 
+    def get_webauthn_credentials_for_user(self, user_id):
+        from datetime import datetime
+
+        return [{
+            'name': 'Ben test',
+            'created_at': datetime.now().strftime("%Y-%m-%dT%H:%M:%S.%fZ")
+        }]
+
 
 user_api_client = UserApiClient()