Studio: Pass custom domain to WP.com onboarding flow

[katinthehatsite]

Related issues

Fixes STU-1096

Proposed Changes

This PR passes the custom domain to WP.com onboarding flow if the user uses the custom domain.

If not, we pass the Studio site name:

Testing Instructions

• Pull the changes from this branch
• Create a Studio site with a custom domain name
• Navigate to the Sync tab in Studio and open the Sync modal
• At the bottom of the modal, click on Create a new WordPress.com site
• Observe that when you are brought to WP.com, the domain name field on WP.com is populated with your Studio site custom domain
• Then, test this flow with a site that does not use a custom domain
• Observe that when you are brought to WP.com, the domain name field on WP.com is populated with your Studio site name

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[github-actions]

📊 Performance Test Results

Comparing 8771a18 vs trunk

site-editor

Metric	trunk	8771a18	Diff	Change
load	7765.00 ms	10476.00 ms	+2711.00 ms	🔴 34.9%

site-startup

Metric	trunk	8771a18	Diff	Change
siteCreation	13430.00 ms	14234.00 ms	+804.00 ms	🔴 6.0%
siteStartup	6955.00 ms	5952.00 ms	-1003.00 ms	🟢 -14.4%

---

Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change

[epeicher]

Thanks @katinthehatsite, this is another touch that improves the UX a bit. I have tested it, and it works as expected. I can see the custom domain when I have one, or the site title. I have left one nit comment about one import, but accepting this so it can be progressed. LGTM!

Custom domain	No custom domain

[sejas]

Works as expected and the customDomain has precedence over the site name. Thanks for working on this issue.
Let's just change the imports to common/constants.