#7686323 Reading Settings: Add new ‘For each new post email, include..’ newsletter setting

Proposed Changes

• introduce new setting field that is responsible for defining whether the current site’s subscription emails containing site’s posts are meant to display full posts’ content, or post excerpts.
• the wpcom_subscription_emails_use_excerpt site option is used to store the setting’s value on the backend

Testing Instructions

The changes are dependent on the /sites/<BLOG-ID>/settings endpoint being able to store and provide the wpcom_subscription_emails_use_excerpt site option. The site option was only recently implemented. The latest release of jetpack does not contain those changes yet. As a result, for testing purposes, we need to sandbox the public-api.wordpress.com with the latest trunk version applied to it.

• Apply jetpack’s trunk to your sandbox
• Sandbox public-api.wordpress.com
• Checkout this branch and run calypso locally
• Go to http://calypso.localhost:3000/settings/reading/<YOUR-BLOG>
• Ensure that the new setting UI behaves as expected, including after page reloads etc. You can help yourself by inspecting GET and POST calls to /site/<BLOG-ID>/settings endpoints.

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)?

Closes #70427