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/helpers/link_list_helper.rb

@@ -3,8 +3,8 @@ module LinkListHelper
     return "" if links.compact.empty?
 
     tag.ul(options) do
-      safe_join(links.compact.map do |text, url, is_active = false, **link_options|
-        tag.li(class: ("is-active" if is_active)) do
+      safe_join(links.compact.map do |text, url, current = false, **link_options|
+        tag.li(({ "aria-current": true } if current)) do
           link_to text, url, link_options
         end
       end)