Onboarding issues experiment welcome screen

What does this MR do?

Modify the signup welcome screen for the onboarding issues experiment. (design)

When in the experiment, the welcome page should:

• show the submit button with the text 'Continue'
• show the progress bar with the following steps:

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

When in the experiment and when buying a subscription, the welcome 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)

Screenshots