Fix label serialisation in issue and note hooks
What does this MR do?
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29384 was not my greatest review. We were not calling hook_attrs on the labels correctly. Specs were
passing because the issues under test did not have any labels! I also fixed the issues hook while I was there as it had a similar issue.
Does this MR meet the acceptance criteria?
Conformity
Performance and testing
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Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/63473.