Self service downgrade: Show refund text for eligible users #17317926

• Related to #100441
• Related to pau2Xa-6Mk-p2

Proposed Changes

• Show refund text in the Downgrade screen: When you downgrade from %(currentPlan)s to %(targetPlan)s, we'll immediately process a refund of %(amount)s to your original payment method.
• Show the Refund available beneath the Downgrade button (similar to Cancel)

Why are these changes being made?

• Part of an epic to support self service downgrades in wpcom

Testing Instructions

• Go to the purchases screen for a plan subscription found within the refund window
• You should see the Refund available text in Downgrade button

• The Downgrade screen should show the refund text

• Switch to a plan outside the refund window
• The Refund available should not be visible
• The Downgrade screen should show the default text

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 UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)? The length of text and words vary significantly between languages.
• 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-aUh-p2)?