Scoped labels follow-up - check feature_availability on issuable's parent

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9983#note_157682499:

There is minor inconsistency: When rendering a label, label's project/group is used for checking if the feature is available. On the other side for frontend we pass one flag if scoped_labels are available on the resource being work with, ans similar when filtering exclusive labels in issuable_base_service we check issuable parent too. I think it's not too big deal (result of license check should be same in both cases unless we allow different license for a project vs its parent groups).

It would be better to change label rendering to use issuable's parent rather than label's parent when checking if scoped_labels feature is available (although result should be same in both cases).