Site Migration: Update Move to WordPress.com plugin to use new site migration flow variant

[sixhours]

The Move to WordPress.com plugin points to the import-focused flow, the code for which can be found in Jetpack.

This reference should be updated to point 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.

[sixhours]

Looks like this is handled by jetpack.com/redirect with a wpcom-migration-handler-route source value. I'm not able to find the value in our searchable repositories so I'll check in with Jetpack folks.

[sixhours]

FG page on JP redirects: PCYsg-pY7-p2 TIL

[sixhours]

Plugin itself does not need to be updated; I've updated the redirect using our internal tool to point to /setup/hosted-site-migration