Add a left-hand navigation to the security page

- create a new layout
- create a new sidebar
- update tests

ee/app/controllers/security/dashboard_controller.rb

@@ -2,5 +2,6 @@
 
 module Security
   class DashboardController < ::Security::ApplicationController
+    layout 'instance_security'
   end
 end

ee/app/views/layouts/instance_security.html.haml

+- page_title _('Security')
+- header_title _('Security'), instance_statistics_root_path
+- nav 'security'
+- @left_sidebar = true
+
+= render template: 'layouts/application'

ee/app/views/layouts/nav/sidebar/_security.html.haml

+.nav-sidebar{ class: ("sidebar-collapsed-desktop" if collapsed_sidebar?) }
+  .nav-sidebar-inner-scroll
+    .context-header
+      = link_to security_root_path, title: _('Security Dashboard'), id: 'logo' do
+        .avatar-container.s40.settings-avatar.rect-avatar
+          = brand_header_logo
+        .sidebar-context-title
+          = _('Security')
+      %ul.sidebar-top-level-items
+        = nav_link(path: %w[dashboard#show]) do
+          = link_to security_root_path, class: 'shortcuts-project rspec-project-link' do
+            .nav-icon-container
+              = sprite_icon('dashboard')
+            %span.nav-item-name
+              = _('Security Dashboard')
+
+    = render 'shared/sidebar_toggle_button'

ee/changelogs/unreleased/230411-security-left-nav.yml

+---
+title: Add a left-hand navigation to the security page
+merge_request: 38529
+author:
+type: changed

ee/spec/controllers/security/dashboard_controller_spec.rb

@@ -3,11 +3,34 @@
 require 'spec_helper'
 
 RSpec.describe Security::DashboardController do
+  let_it_be(:user) { create(:user) }
+
   describe 'GET #show' do
+    subject { get :show }
+
     it_behaves_like Security::ApplicationController do
       let(:security_application_controller_child_action) do
         get :show
       end
     end
+
+    context 'when security dashboard feature' do
+      before do
+        sign_in(user)
+      end
+
+      context 'is enabled' do
+        before do
+          stub_licensed_features(security_dashboard: true)
+        end
+
+        it { is_expected.to render_template(:instance_security) }
+      end
+
+      context 'is disabled' do
+        it { is_expected.to have_gitlab_http_status(:not_found) }
+        it { is_expected.to render_template('errors/not_found') }
+      end
+    end
   end
 end