Follow-up from Add group labels

Follow-up from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6425

• ~P1 Support subscribing to group-level labels
  • https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6425#note_17066906
  • https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6425#note_17147374

• Ensure all Referable methods on {Group,Project}Label support the correct, expected signature by using keyword arguments in the override, or redefining the methods on Referable ~"technical debt"
  • https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6425#note_17066151
  • https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6425#note_17066261
• Add proxy class to make it easier to get/set project-dependent properties of GroupLabel ~"technical debt"
  • https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6425#note_17066872
• Prefer polymorphism (or by editing Referable for everyone) over referable.respond_to?(:project) ~"technical debt"
  • https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6425#note_17067245
• ~~Define label and labels as aliases in import/export configuration, if possible ~"technical debt"~~ See: https://gitlab.com/gitlab-org/gitlab-ce/issues/23586#note_21673641
  • https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6425#note_17169536