GitLab

The public settings API filters out menu items with visibility='admin',
so customMenuItemsForAdmin was always empty when reading from
cachedPublicSettings. Fix by loading custom menu items from the admin
settings API (via adminSettingsStore) which returns all items unfiltered.

Changes:
- adminSettings store: store custom_menu_items from admin settings API
- AppSidebar: read admin menu items from adminSettingsStore instead of
  cachedPublicSettings
- CustomPageView: merge public and admin menu items so admin users can
  access admin-only custom pages