Subgroup support for issues, merge requests, labels, and milestones
### Background and current implementation
- Issues are scoped at the project level.
- Labels are scoped at the project level _and_ group level.
- Milestones are scoped at the project level _and_ group level.
- Currently, despite having subgrouping functionality, we do not support visibility of issues being rolled up beyond their immediate parent group.
| | Current | Proposed |
| --- | --- | --- |
| Assign label to issue/mr (issue/mr page, board sidebar) | Assign project label, immediate parent group label | Expand to all ancestor group labels https://gitlab.com/gitlab-org/gitlab-ce/issues/40915 |
| Assign milestone to issue/mr (issue/mr page, board sidebar) | Assign project milestone, immediate parent group milestone | Expand to all ancestor group milestones https://gitlab.com/gitlab-org/gitlab-ce/issues/36862 |
| Group list views of issues/mrs | View objects of current group's immediate children projects | Expand to objects of all projects of all descendent groups https://gitlab.com/gitlab-org/gitlab-ce/issues/30106, https://gitlab.com/gitlab-org/gitlab-ce/issues/39388, https://gitlab.com/gitlab-org/gitlab-ce/issues/42443|
| Group board view of issues | View issues of current group's immediate children projects | Expand to issues of all projects of all descendent groups https://gitlab.com/gitlab-org/gitlab-ee/issues/3342 |
| Group list filtering, group board filtering, group board saved config, group board lists of issues/mrs by labels/milestones | Filter by labels/milestones of current group and immediate children projects | Expand to labels/milestones of all ancestor/descendent groups https://gitlab.com/gitlab-org/gitlab-ce/issues/40915, https://gitlab.com/gitlab-org/gitlab-ce/issues/44723, https://gitlab.com/gitlab-org/gitlab-ce/issues/36862 |
epic