Subscription hooks: improve logged in state

When logged in and subscribed, the subscribe block hook at the end of the post signals you're subscribed:

When site has paid subscriptions defined, we'd need to show a nudge (copy + CTA) similar to paywalls for paid posts when logged in:

For sites without paid subscriptions, we could consider hiding? Even then it might be important to keep showing admins so that they see how it'll look like even if they are subscribed.

Start by locating the subscribe block hook at the end of a post and compare its current logged-in, subscribed output with the paid-post paywall behavior. Define the logged-in experience for sites with paid subscriptions, including the copy and CTA, and decide whether non-paid sites hide the block while admins still see it. Done means each described subscription state renders the intended result.

Written by the indexing model from the issue text.