Show issues of subgroups in group-level issue board

• In the current implementation of group issue boards (https://gitlab.com/gitlab-org/gitlab-ee/issues/928), we only show issues across all projects of that group.
• It would be vastly helpful to show issues of projects that belong to subgroups of that particular group.
• We need to figure out and implement labels from ancestor groups before doing this one, to make it consistent, i.e. &22 (closed)
• We should also maintain API consistency for this issue too.

Consider:

• group-a/group-aa/group-aaa/project-1
• group-a/group-aa/group-aab/project-2
• group-a/group-aa/group-aac/project-3
• group-a/group-ab/group-aba/project-4
• group-a/group-ac/project-5
• group-a/project-6

A group board for group-a should contain all issues in all project-1, project-2, project-3, project-4, project-5, project-6.

A group board for group-aa should contain all issues in all project-1, project-2, project-3.

Issue board card path: