#7931457 Signup: Add free plan + paid domain modal

Fixes Automattic/martech#1649

Proposed Changes

The full spec is in Automattic/martech#1649.

Domains Step

• Change in the free domain explainer copy:

Plans Step

• Change in the domain feature copy for paid plans:

• Change in the domain feature copy for the free plan. The .wordpress.com subdomain is fetched via a new hook useGetWordPressSubdomain.

• The free plan + paid domain modal when a paid domain has been selected, and the user clicks on the Free plan CTA. This uses the same hook, and the value is cached for 5 minutes, so in most cases, a separate network is not required to fetch the subdomain.

Testing Instructions

1. Go to /start.
2. On the domains step, search for some domains. Confirm that the free-domain explainer text (in green) matches the spec.
3. Select a paid domain.
4. On the plans step, confirm that the domain feature for paid plans matches the spec.
5. Confirm that a wordpress.com subdomain is displayed under the crossed-out domain name in the feature list of the free plan.
6. Click on the Start with Free CTA.
7. Confirm that the modal is shown, and the subdomain matches the one shown on the plans grid.
8. Confirm that the price shown for the Personal plan is correct.
9. Select Get Personal, and confirm that you are taken to the checkout page with the selected paid domain and the Personal plan in the cart.
10. Repeat steps 1-8.
11. Select “Continue with Free”.
12. Confirm that a free site is created with free subdomain shown on the modal.

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)?