[Onboarding/React] Track progress (next-buttons) in getting started flow

At the moment when refreshing the page, the getting started flow loses where I was in the progress, causing me to click next a few times to get back where I was. This is especially annoying when I'm refreshing to see if maybe my error has arrived, and then having to go through all the steps again.

Let's put it in the URL so that it's preserved when you refresh the page.