Alert the user when their Premium auth token becomes invalid (401 status) and log them out of Premium

[mitchdowney]

It seems like users are sometimes mysteriously no longer able to use Premium features, even though they have a valid Premium membership, and the app thinks they are logged-in.

One thing that could possibly help is, when a logged-in user receives a 401 response, then we can assume that user no longer has a valid logged-in session, so we can render a "You have been logged out of Podverse. If you would like to resume using Podverse Premium, please log back in..." message, and then remove the auth token from local storage, so the app doesn't get confused by thinking the user still has a valid auth token.

[mitchdowney]

I think I have the correct code for this in #1470

But ALL our alerts are broken in v4.8.0 because of the same type of problem described in facebook/react-native#34562