CE counterpart of: Namespace license checks for multiple assignees

What does this MR do?

This ports back the changes of gitlab-org/gitlab-ee!1916 so future merges will/should? result in less conflicts.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Related to gitlab-org/gitlab-ee#2335.