Subscription "email verification" wall for paid posts should still allow upgrading

When reader subscribes to the blog for free, and doesn't verify their email, we show "verify your email" instead of the subscription paywall.

That's fine for free posts, but for paid walls we could continue showing the paywall _and_ the subscription, thus allowing them to upgrade to paid plan on the spot.

It's a bit more minor situation, so the solution doesn't need to be very novel, but curious what we can think of. Perhaps we just show subscribe block inside the "verify your email" banner and it'll render as "Upgrade to paid" on its own? @crisbusquets thoughts?

Search the subscriptions/paywall and email-verification flows in the Jetpack repository, then trace the path for a reader who has not verified their email. Compare the current behavior for free and paid posts and locate the existing subscription or upgrade entry point; done means the paid wall and upgrade action remain available while verification messaging is shown.

Written by the indexing model from the issue text.