New onboarding: MVC

This is a placeholder issue for the first iteration of our ~"product discovery" in https://gitlab.com/gitlab-org/gitlab-ce/issues/55523 on new onboarding.

Please evaluate the discovery and participate in https://gitlab.com/gitlab-org/gitlab-ce/issues/55523. You can see a prototype of the proposed solution in https://sketch.cloud/s/boO9j/OmQ7w18/play.

The goal of this first iteration should be:

• Get a majority of new users to start the onboarding.
• Get a majority of onboarding starters to complete the onboarding.
• Improve second session return rates among onboarding completers.