Handle expiry of tokens

Pixelfed makes the accessToken expire after 15 days. We haven't noticed this since we reset the app quite often, of course.

There is a way to refresh the token programmatically (no user intervention) with a refresh token: https://laravel.com/docs/7.x/passport#refreshing-tokens, this can be done until 30 days after authentication. This also gives a new refresh token, although I'm not sure if that one is valid 30 days starting at the refresh or not.

If we can't get a new token automatically, we should present the OAuth prompt to the user again.