Only show outline on submenu toggle when activated by keyboard

Avoid showing a focus ring on the item when activating it via
touch/mouse for consistency with how other tab-reachable parts of the
menu and top bar work.

src/styles/sidebar/components/menu-item.scss

@@ -91,6 +91,8 @@ $menu-item-padding: 10px;
 // Toggle button used to expand or collapse the submenu associated with a menu
 // item.
 .menu-item__toggle {
+  @include outline-on-keyboard-focus;
+
   display: flex;
   flex-direction: column;
   justify-content: center;