Protected Audit Log page behind labs flag

refs https://github.com/TryGhost/Toolbox/issues/356

- even though I was hiding the button, it's better to prevent going to
  the route entirely by redirecting to the homepage if the labs flag is
  not enabled

ghost/admin/app/routes/settings/audit-log.js

@@ -1,6 +1,16 @@
 import AdminRoute from 'ghost-admin/routes/admin';
+import {inject as service} from '@ember/service';
 
 export default class AuditLogRoute extends AdminRoute {
+    @service feature;
+
+    beforeModel() {
+        super.beforeModel(...arguments);
+        if (!this.feature.auditLog) {
+            return this.transitionTo('home');
+        }
+    }
+
     buildRouteInfoMetadata() {
         return {
             titleToken: 'Settings - Audit log'