Add joining_project to the onboarding_status jsonb column

What does this MR do and why?

• keep track of user decisions during onboarding better
• see #498743 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After
	no change

How to set up and validate locally

note all covered by ee/spec/features/registrations/saas/*

1. Setup to simulate SaaS and restart GDK
   • Make sure Admin > Settings > General > Sign Up Restrictions match the below (no admin approval, but hard email confirmation)
2. Register as a new user.
3. Confirm email by finding the confirmation path from last User created in rails console
   • Rails.application.routes.url_helpers.user_confirmation_path(confirmation_token: User.last.confirmation_token)
4. Visit the path from the previous step.
5. You should be on the sign in page, log back in.
6. Select any 'role' and 'Just me' in the welcome form after sign up step.
7. Choose 'Create a new project' - this is the field that is now in the db
8. Click 'Continue'
9. Fill out fields to create a group and project on the next page.
10. Click 'Create project'
11. Click on "Ok, let's go" and verify you are dropped into the learn gitlab project and the celebrate modal pops up with confetti

Related to #498743 (closed)