Site Migration: Update import-focused reference from /sites

[sixhours]

Via the site picker at /sites: Click on the arrow next to Add new site -> Import an existing site.

The code for this hook is here: fbhepr%2Skers%2Spnylcfb%2Spyvrag%2Sfvgrf%2Qqnfuobneq%2Subbxf%2Shfr%2Qfvgrf%2Qqnfuobneq%2Qvzcbeg%2Qfvgr%2Qhey.gf%3Se%3Q6pr8rr1p-og

Once the new site migration flow variant is live we need to update this reference.

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]

This was merged in #90904