Gifting: show gifted site name in Checkout

[pottedmeat]

Proposed Changes

• Adds gift_details to ResponseCart (introduced in D90977-code)
• Updates the way the displayed site is determined
• Prioritizes using a domain name in the cart
• Uses receiver_blog_url from this response as a fall-through before using siteUrl

Testing Instructions

You'll need two accounts and make sure that the store is sandboxed.

• With account A, purchase a personal plan (or use an account which already owns a plan) and a .live domain name
• With account B, go to the following URL: /checkout/personal-bundle/gift/<ACCOUNT_A_SUBSCRIPTION_ID>. You can use Store Admin to find the subscription ID of the account A's plan.
• Verify the site name is account A's blog URL
• 
• With account B, go to the following URL: /checkout/personal-bundle,dotlive_domain/gift/<ACCOUNT_A_PLAN_SUBSCRIPTION_ID>,<ACCOUNT_A_DOMAIN_SUBSCRIPTION_ID>. You can use Store Admin to find the subscription ID of the account A's plan and .live domain.
• Verify the site name is the domain name added to the purchase
• 

Fixes #1195

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:commit-dfb6801fe52541b4b29a3147f677e092a56dd58b

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:commit-dfb6801fe52541b4b29a3147f677e092a56dd58b&env=jetpack

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Sections (~26 bytes added 📈 [gzipped])

name      parsed_size           gzip_size
checkout        +36 B  (+0.0%)      +26 B  (+0.0%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Async-loaded Components (~26 bytes added 📈 [gzipped])

name                                             parsed_size           gzip_size
async-load-calypso-my-sites-checkout-modal             +36 B  (+0.0%)      +26 B  (+0.0%)
async-load-calypso-blocks-editor-checkout-modal        +36 B  (+0.0%)      +26 B  (+0.0%)

React components that are loaded lazily, when a certain part of UI is displayed for the first time.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[sirbrillig]

Looks good to me!