Use aria-current to mark the current element

This way screen reader users will be notified that the element is the
current one.

I'm not entirely sure whether `aria-current="page"` is more appropriate
than `aria-current="true"`, since it's a general helper which can be
used for any collection of links.

app/assets/stylesheets/admin/menu.scss

@@ -117,7 +117,8 @@
       padding-left: $line-height / 2;
     }
 
-    &.is-active a {
+    &.is-active a,
+    &[aria-current] a {
       background: $sidebar-hover;
       border-left: 2px solid $sidebar-active;
       font-weight: bold;