Issue/epic automatically has labels from ancestor epics/issues

Background

• Epics of epics - &312 (closed)
• Issues of issues - https://gitlab.com/gitlab-org/gitlab-ee/issues/2036
• Epics can already contain issues

Proposal

• Suggested by @annabeldunstone https://gitlab.com/gitlab-org/gitlab-ee/issues/7077#note_111961242.
• Child object automatically has labels of parent object and all ancestors.
• All ancestors means that if you have a very tall tree, then an object gets the union of all labels of all ancestors.
• You cannot change these automatically "inherited" labels. There's a section in the sidebar of the object, showing these inherited labels.
• What happens is that when you do any type of searching/filter (Web, API) with labels, then this object would be captured for labels from its ancestors.