Add "Create your first project" registration step to SM

What does this MR do and why?

Adds create first project onboarding for self-managed installations

Implements a "Create your first project" registration step that appears for the root admin user on their second login to a new self-managed GitLab instance.

This feature:

• Redirects admins to project creation flow on second sign-in
• Only triggers when sign_in_count reaches 2 (after second login)
• Requires exactly one human user on the instance
• Ensures admin has no existing groups
• Guides users through creating their first group and project

The onboarding helps new self-managed administrators get started by walking them through creating their first project to store code.

The implementation adds:

• after_sign_in_path_for override in SessionsController
• New admin/registrations/groups controller and routes
• Project creation view reusing existing components
• Comprehensive test coverage for all conditions

References

• https://gitlab.com/gitlab-org/gitlab/-/work_items/579942

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Make sure that GitLab is not setup to simulate saas (that this env var is not set GITLAB_SIMULATE_SAAS=1)

2. Set your root user sign_in_count = 1 in the rails console and save your user

3. Quickstart: comment out the following lines or return true

   def should_show_self_managed_project_onboarding?(user)
     return false unless user.can_admin_all_resources?
     return false unless user.sign_in_count == 2
     # return false unless User.human.one?
     # return false if user.owned_groups.any?
   
     true
   end

4. Create group from admin/sign_up/groups/new

MR acceptance checklist

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Related to #579942