Allow listing direct members only in group members page

Problem to solve

Mentioning a subgroup only mentions direct members: https://docs.gitlab.com/ee/user/group/subgroups/#mentioning-subgroups

For instance, mentioning @gitlab-org/maintainers/rails-backend does not mention all members of @gitlab-org, only the people who are explicitly added to that subgroup. This is a very useful feature! But, it's hard to know who's going to be mentioned when you type @gitlab-org/maintainers/rails-backend, because the members list shows all users, including those with inherited access: https://gitlab.com/groups/gitlab-org/maintainers/rails-backend/-/group_members

Intended users

Anyone who administers or @-mentions groups.

Proposal

We should add a filter on the group members page to see who's a direct member.

Permissions and Security

This should use the same permissions as visiting the group page.

Documentation

https://docs.gitlab.com/ee/user/group/subgroups/#membership

Testing

This should be a UI change to expose the internal database structure more clearly.

What does success look like, and how can we measure that?

I don't know how to answer this question. I think implementing this feature would be success in itself; we'd reduce the confusion over where people get their access. We could measure it anecdotally by tracking questions in the GitLab Slack (I think this has come up three times this year that I've seen, and I don't read everything in our Slack)?