Add a supporting code to separate groups#show
and groups#details which is required for the
proper implementation of the Group Overview
content and Security Dashboard option for it

- we now use the hierarchy class also for epics
- also rename supports_nested_groups? into supports_nested_objects?
  - move it to a concern

With this MR, group clusters is now functional, so default to enabled.

Have a single setting on the root ancestor group to enabled or disable
group clusters feature as a whole

As the frontend is still in progress and other related backend is still
in prgress we need a feature flag to turn off Group Clusters.

As this is the only item in this menu, make Kubernetes the link for the
sidebar item too.

- Allow to extend nav links for the Group sidebar

Partially addresses #47424.

This allows us to check specific abilities in views, while still
enabling/disabling them at once.

Previously, these wouldn't count issues or MRs in subgroups - meaning that if
_this_ group had no issues or MRs, we'd show the empty state, which was wrong.

Tests for these new helper methods

Closes #37745

…based on whether the current user owns the parent group and whether the parent group or current group is share locked.

fixed up Ruby based on review