Move welcome controller to ee from ce

What does this MR do and why?

Moves the welcome controller to ee only as it is only needed there now that ce does not have any onboarding. Use of route removed from foss in !130335 (merged)

Step 2 in the plan

Screenshots or screen recordings

no change

How to set up and validate locally

Non SaaS

1. Ensure GDK is not setup to simulate SaaS
2. Disable check_namespace_plan in rails console
   • ApplicationSetting.first.update(check_namespace_plan: false)
3. Restart GDK
4. Register as a new user.
5. After registration, and any confirmation steps, you should land on the dashboard/projects page.

SaaS (no behavior change)

1. Ensure GDK is setup to simulate SaaS
2. Enable check_namespace_plan in rails console
   • ApplicationSetting.first.update(check_namespace_plan: true)
3. Restart GDK
4. Register as a new user.
5. After registration, and any confirmation steps, you should land on the welcome page(in onboarding).

note You can get the new user confirmation token, if you setup is set that way, by running the below in rails console

• Rails.application.routes.url_helpers.user_confirmation_path(confirmation_token: User.last.confirmation_token)

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 #411858 (closed)