Frontend: Scaffold group-level Security configuration page

Scaffold out a blank-ish Category label configuration screen:

🎨 design: #547963[Security_configuration_-_group-level_-_security_labels_-_category_details_-_full_edit.png]

• 🧵 Do we need permission to add this to the sidebar?
  • no, it's probably more confusing that this page doesn't exist at the group level already
• 🧵 Should this view be shown only for the root group?
  • no, let the user (if they have permission) edit the set of labels that belongs to the root group from any subgroup within it
• 🧵 What role/permission(s) does a user need to have to create and edit labels?
  • to create/edit/delete security labels, a user will need a new permission
  • a user that can view a project can view its security labels

---

• introduce a feature flag security_context_labels that performs its enabled? checks on the root_ancestor of a group or project
• create a feature flag roll-out issue: [FF] `security_context_labels` -- Categorize pr... (#551226)
• add a licensed feature security_labels to ee/app/models/gitlab_subscriptions/features.rb in the ULTIMATE_FEATURES section (different from feature flag to prevent confusion)

---

• add Secure => Security configuration to the group sidebar
  • only show if user has sufficient permissions, licensed feature is available, and feature flag is enabled
• link it to [GROUP URL]/-/security/configuration (same path as project level)

---

• create a page at [GROUP URL]/-/security/configuration
  • render 403 if user has insufficient permissions, licensed feature is unavailable, or feature flag is disabled
• add a header with title Security configuration
• add a tab bar containing one tab: Security labels
• add description text

---

• frontend review
• frontend maintainer review
• backend review
• backend maintainer review
• groupauthorization maintainer review (required for /ee/app/policies/ changes)