#7932034 Subscriptions management: Implement searching for Sites (subscriptions) list

Resolves #75705

Proposed Changes

• Implement case-insensitive search for Sites list
• The search is executed on the frontend
  • It depends on all the site subscriptions data being loaded from the backend
• I also moved reader mutation tests into /packages/data-stores/src/reader/mutations/tests because they were not being executed as part of the yarn run test-packages test run when they were in their previous location

Testing Instructions

1. Check out the PR and build it locally.
2. Get the subkey cookie by sending yourself the confirmation email at https://wordpress.com/email-subscriptions and clicking the verification link in the Incognito browser window:

3. Navigate to http://calypso.localhost:3000/ and set the subkey cookie:

4. Make sure the External user you test with, has multiple subscriptions.

If you like, you can use the https://public-api.wordpress.com/rest/v1.2/read/site/SITE_ID/post_email_subscriptions/new endpoint to quickly subscribe to multiple sites:

Please note that SITE_ID represents the site you are subscribing to and the Authorization header needs to match the subkey cookie value in the form of X-WPSUBKEY subkey_value.

5. Navigate to http://calypso.localhost:3000/subscriptions/sites and try out the search input box.

(Credit for the test instructions goes to @ivan-ottinger – #75115)

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)?