#17316057 Self service downgrade: Allow lossless reverts

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

Proposed Changes

This adds a checkbox that triggers lossless reverts to recover posts, pages and media added after the site became atomic.

Why are these changes being made?

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

Testing Instructions

• Locally there needs to be a feature flag activated plans/self-service-downgrade, flags=plans/self-service-downgrade

• Sandbox public-api.wordpress.com with 178193-ghe-Automattic/wpcom
• Go to a site with a Business plan, which is atomic with the default wpcomstaging url
• Ensure you have a Post published after the site became atomic
• Go to /purchases/subscriptions/<wpcomstaging.com-site-slug>
• Click on the downgrade button visible at the purchase manage page
• The next screen should also show a checkbox to recover posts, pages, and media content: make sure to check it as well and click the CTA
• The downgrade should work and the site should be added to the lossless revert queue which you can check in 37583-pb/#bash
• You may need to wait for the site to reach complete state. Once done, go to the Posts screen where you should see you recovered post

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)?