#7901484 Launchpad :: add newsletter steps (stripe/paid subscriptions)

Related to #72651, P2: p81Rsd-1dU-p2

Proposed Changes

• Add stripe account connected step to newsletter flow.
• Add paid newsletter plan to newsletter flow.

Screenshot

See “Connect Stripe account” and “Create a paid newsletter”.

Testing Instructions

• Checkout this branch
• Sandbox public-api.wordpress.com & store: PCYsg-IA-p2.
• Start a new newsletter site from – http://calypso.localhost:3000/setup/newsletter/intro
• Checkout
• Verify that the steps now include connecting a stripe account and creating a paid plan.
• Following those steps should mark off the corresponding options.
• Returning from the stripe flow kicks you onto wordpress.com. You will have to mangle the URL to return to your localhost environment — which won’t happen once this PR is merged. (i.e. http://calypso.localhost:3000/earn/payments/${siteSlug}?stripe_connect_success=launchpad). Verify that the url param is correct.

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the “[Status] String Freeze” label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• For changes affecting Jetpack: Have we added the “[Status] Needs Privacy Updates” label if this pull request changes what data or activity we track or use (p4TIVU-ajp-p2)?