Add active highlight for nav item

app/views/layouts/nav/sidebar/_group.html.haml

@@ -2,6 +2,7 @@
 - merge_requests = MergeRequestsFinder.new(current_user, group_id: @group.id, state: 'opened', non_archived: true).execute
 - epics = EpicsFinder.new(current_user, group_id: @group.id).execute
 
+- epics_items = ['epics#show', 'epics#index']
 - issues_sub_menu_items = ['groups#issues', 'labels#index', 'milestones#index']
 - if @group.feature_available?(:group_issue_boards)
   - issues_sub_menu_items.push('boards#index', 'boards#show')
@@ -45,7 +46,7 @@
                   Contribution Analytics
 
       -# TODO: Add the flag check to only show epics if available
-      = nav_link(path: 'groups#epics') do
+      = nav_link(path: epics_items) do
         = link_to group_epics_path(@group) do
           .nav-icon-container
             = sprite_icon('epic')
@@ -53,7 +54,7 @@
             Epics
           %span.badge.count= number_with_delimiter(epics.count)
         %ul.sidebar-sub-level-items.is-fly-out-only
-          = nav_link(path: 'groups#epics', html_options: { class: "fly-out-top-item" } ) do
+          = nav_link(path: epics_items, html_options: { class: "fly-out-top-item" } ) do
             = link_to group_epics_path(@group) do
               %strong.fly-out-top-item-name
                 #{ _('Epics') }