Move onboarding to learn gitlab controller

What does this MR do and why?

What

• Moves onboarding to the learn gitlab controller

Why

• That action more closely aligns with the goals of the learn gitlab controller and moving it helps us simplify the welcome controller responsibilities.

Screenshots or screen recordings

no change

How to set up and validate locally

note basically trying to emulate feature spec steps in ee/spec/features/registrations/saas/standard_flow_just_me_creating_project_spec.rb

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'
8. Click 'Continue'
9. Fill out fields to create a group and project on the next page.
10. Click 'Create project'
11. Verify you get to the onboarding and page refreshes to provide a "Ok, let's go" button.
12. Click on "Ok, let's go" and verify you are dropped into the learn gitlab project and the celebrate modal pops up with confetti

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #387662 (closed)