"Label" & "Milestones" for groups moves out of "Issues", into "Epics"
Epics will be for groups the main source to manage, while issues are so for projects. In projects labels and milestones are represented under issues, however for groups epics are the highest level.
Let's move labels and milestones into the section of epics.
No duplication, similar as to https://gitlab.com/gitlab-org/gitlab-ce/issues/23134