Problemset namespace

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Use problemset grouping like GitHub/GitLab repositories. Each repository belongs to a namespace, which could be a user namespace or an organization namespace.

Gitlab references:

• https://gitlab.com/gitlab-org/database-team/team-tasks/uploads/62c7b49b943cb2396dd17c6323acd6c1/GitLab.com_ERD_-_2020-11-05.pdf
• https://dbdiagram.io/d/5fb7f8f53a78976d7b7cc6fb
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/db/structure.sql
• https://azimutt.app/gallery/gitlab