GH-2033 Update GBE account state in response to checkout-web events

[wlycdgr]

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Uses messages broadcast by checkout-web, picked up by an injected content script, and handled by the background script, to:

• Automatically log user in, fetch their settings (if their email is validated), and fetch their subscription data if they log into their account on checkout-web
• Automatically log user in if they register on checkout-web
• Automatically update user's subscription status if they buy Plus, Premium, or Insights on checkout-web