Site Migration: Update import-hosted-site ref from /hosting landing page

[sixhours]

The hosting landing page (https://wordpress.com/hosting/) links to /setup/import-hosted-site when clicking the Migrate Site button.

This reference needs to be updated to the new site migration flow variant once it's live.

We should also ensure there's a ref query param added to the URL with a unique string value so we can track where users are coming into the new flow from.