Admin area: Group details has no list of subgroups

Description

In admin area, the group details page displays a list of projects in that group, but no list of sub-groups.

Proposal

Also add a list of subgroups to that details page to allow faster navigation and to see if a group has other nested elements.

Links / references

Documentation blurb

Overview

What is it? Why should someone use this feature? What is the underlying (business) problem? How do you use this feature?

Use cases

Who is this for? Provide one or more use cases.

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.