New user onboarding (Skeleton)

What does this MR do?

This is one of many small MRs for the new user onboarding feature, see https://gitlab.com/gitlab-org/gitlab-ce/issues/60093.

It adds the skeleton behind the :user_onboarding feature flag and renders the wrapping container for the onboarding helper on every page. This container will be used to mount the Vue app. The Vue app will be part of a separate MR.

CE Port: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28753

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