New onboarding launch: use site slug instead of siteId when redirecting

[razvanpapadopol]

Changes proposed in this Pull Request

• Use site slug instead of siteId when redirecting to /checkout.
• Use site slug when redirecting to home after launching the site.

Testing instructions

• Create a site starting from /new
• Sync ET plugin and sandbox the site
• Start Launch from the editor and complete the flow by choosing paid plan => redirect to /checkout/{SITE_SLUG} with no errors when loading cart (see Launch: errors after redirect to checkout #47150).
• Repeat previous steps but choose a Free plan instead => redirect to /home/{SITE_SLUG}

Fixes #48091
Fixes #47150

[matticbot]

Test live: https://calypso.live/?branch=update/new-onboarding-launch-redirect-using-site-slug

[matticbot]

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

Async-loaded Components (~8 bytes added 📈 [gzipped])

name                                           parsed_size           gzip_size
async-load-calypso-blocks-editor-launch-modal        +60 B  (+0.0%)       +8 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.

[matticbot]

Caution: This PR affects files in the Editing Toolkit Plugin on WordPress.com
Please ensure your changes work on WordPress.com before merging.

D54158-code has been created so you can easily test it on your sandbox. See this FieldGuide page about developing the Editing Toolkit Plugin for more info: PCYsg-ly5-p2

[tjcafferkey]

Tested as described and changes LGTM