Domains: Allow bulk domain transfer of domains that are already connected to a WPCOM site

[leonardost]

Proposed Changes

This PR updates the bulk domain transfer flow to allow transferring domains that are already connected to a WPCOM site.

Transferring already connected domains wasn't possible in that flow because only the transferrable availability status was being considered - that's correct, but we want to make it easier to transfer domains to us.

Screenshots

• Before

• After

Testing Instructions

• Open the live Calypso link or build this branch locally
• Connect an external domain you own to a WPCOM site
• Unlock that domain and get its auth code
• Go to /setup/bulk-domain-transfer/domains
• Input that domain and its auth code in the form
• Ensure it's validated correctly and you are able to proceed

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)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-72351

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-72351&env=jetpack

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[hambai]

Okay, I'll approve that temporary. In the mean time I'll work on a proper fix where we expose id a domain is transferrable in a separate property