Google sign-in
https://developers.google.com/news/subscribe/guides/implement-google-sign-in?authuser=1
Account linking
https://developers.google.com/news/subscribe/guides/oauth-2.0-flow?authuser=1
Buy flow
https://developers.google.com/news/subscribe/guides/sell-subscriptions-on-web?authuser=1
Deferred account creation
What happens if the guy is already logged in?
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We don't need a provisional token, but still need to make API call to publisher.
Account linking on the web
https://developers.google.com/news/subscribe/guides/account-linking-on-web?authuser=1
