Make Codeowners work for subgroups
Problem to solve
Many users including GitLab use subgroups to organize members.
While we can set subgroups for approval rules, right now, we cannot do this for codeowners.
User experience goal
Based on the fact that you can add groups, users expect to be able to add subgroups as well.
Customer ticket example (internal): https://gitlab.zendesk.com/agent/tickets/159432
Once implemented, update codeowners page: https://docs.gitlab.com/ee/user/project/code_owners.html