Allow to find labels in ancestor groups and better group support in label service

What does this MR do?

• It allows to find labels also in ancestor groups if include_ancestor_groups parameter is used.
• Labels::FindOrCreateService can now create labels in groups too (not only projects).
• It allows to pass available_labels as an optional parameter to Labels::FindOrCreateService so FindOrCreateService uses same list of labels as IssuableBaseService (which is useful if a service overrides it - e.g. epics create/update service).

Why was this MR needed?

When listing available labels for epics, we want to list also labels from subgroups - see gitlab-ee#3727