Onboarding app component

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 Onboarding App (including the Vuex implementation) which uses the Onboarding Helper introduced in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13922

The tour content for tour_parts.js is added in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13390

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