This performs liveliness checks on the auth pages to try and show a friendlier error. Earlier checks in the app startup are expected to not block the app from loading on such failures.
See https://github.com/vector-im/riot-web/issues/9828
The app is expected to flag a particular config themselves as default. This is primarily intended so that other parts of the app can determine what to do based on whether or not the config is a default config.
See https://github.com/vector-im/riot-web/issues/9290
