Onboarding issues experiment checkout progress bar

What does this MR do?

Modify the checkout page progress bar for the onboarding issues experiment.

When in the experiment, the checkout page should show the progress bar with the following steps:

1. Your Profile
2. Checkout
3. Your GitLab group
4. Your first project

Parent issue: gitlab-org/growth/product#107 (closed)

There is no changelog, since all changes here are behind a feature flag.

Screenshots