Add onboarding_status to user_details and implement use

- add store for onboarding concerns that we want to record
  in order to either analyze on the data side or for future
  triggering of user experiences outside of onboarding in the
  app.
- eventual replacement of `onboarding_step_url` column.
- see https://gitlab.com/groups/gitlab-org/-/epics/11768

Changelog: other