style: add gicon-shield to sidemenu class

Closes #15591

public/sass/base/_icons.scss

@@ -212,6 +212,9 @@
   .gicon-explore {
     background-image: url('../img/icons_dark_theme/icon_explore.svg');
   }
+  .gicon-shield {
+    background-image: url('../img/icons_dark_theme/icon_shield.svg');
+  }
 }
 
 .fa--permissions-list {