New user onboarding (Welcome screen)

What does this MR do?

Part 1 of the user onboarding (welcome screen) for https://gitlab.com/gitlab-org/gitlab-ce/issues/60093

It adds the Learn Gitlab item to the help menu and redirects the user to the welcome screen where he can start the guided tour. The guided tour is not part of this MR. It will be implemented in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/11852.

This feature is behind the :user_onboarding feature flag and needs to be enabled first.

CE Port: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28062

Screenshots

Menu item	Welcome page

Local Setup

1. Enable feature flag via rails console:

Feature.enable(:user_onboarding)

2. Make sure to change get_onboarding_demo_project in onboarding_controller.rb to the following:

def get_onboarding_demo_project
  Project.find_by_full_path("gitlab-org/gitlab-test")
end

Also change user_onboarding_enabled? in ee/app/helpers/ee/application_helper.rb to the following:

def user_onboarding_enabled?
  ::Feature.enabled?(:user_onboarding)
end

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team