Introduce Issuable#root_id

Which will recursively find the root namespace id, solving gitlab-org/quality/triage-ops#292 (comment 208750292)

Demo at gitlab-org/quality/triage-ops!279 (merged), dry-run: https://gitlab.com/gitlab-org/quality/triage-ops/-/jobs/282473732

Another dry-run: https://gitlab.com/gitlab-org/quality/triage-ops/-/jobs/283039637