Rake task for checking orphaned object store uploads
There was an issue (https://gitlab.com/gitlab-org/gitlab-ce/issues/45425) which caused that object store uploads were not deleted when deleting its parent resource (e.g. when deleting a project). Although https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18698 fixes this issue, we may need to provide a rake task which would print a list of existing objects stored in object store which are not referenced by any upload in DB. This is probably most relevant to gitlab.com
(which is the primary "canary" for object store usage).
/cc @smcgivern