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;

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)

spec/helpers/link_list_helper_spec.rb

@@ -41,7 +41,7 @@ describe LinkListHelper do
       )
 
       expect(page).to have_css "li", count: 3
-      expect(page).to have_css "li.is-active", count: 1, exact_text: "Info"
+      expect(page).to have_css "li[aria-current='true']", count: 1, exact_text: "Info"
     end
 
     it "allows passing both the active condition and link options" do
@@ -52,7 +52,7 @@ describe LinkListHelper do
       )
 
       expect(page).to have_css "li", count: 3
-      expect(page).to have_css "li.is-active", count: 1, exact_text: "Info"
+      expect(page).to have_css "li[aria-current='true']", count: 1, exact_text: "Info"
 
       expect(page).to have_css "a.root", count: 1, exact_text: "Home"
       expect(page).to have_css "a#info", count: 1, exact_text: "Info"