Cleaning up our navigational order - Groups

What does this MR do?

Covers Group nav changes from #26348 (closed)

Why was this MR needed?

See meta issue #25386 (closed) and the related issue #26348 (closed)

Screenshots

Group tab - GitLab CE

Group tab - GitLab EE

Issues tab - GitLab CE/EE

Merge Requests tab - GitLab CE/EE

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

See #26348 (closed)

app/views/groups/_head_issues.html.haml

+= content_for :sub_nav do
+  .scrolling-tabs-container.sub-nav-scroll
+    = render 'shared/nav_scroll'
+    .nav-links.sub-nav.scrolling-tabs
+      %ul{ class: (container_class) }
+        = nav_link(path: 'groups#issues', html_options: {class: 'home'}) do
+          = link_to issues_group_path(@group), title: 'List' do
+            %span
+              List
+
+        = nav_link(path: 'labels#index') do
+          = link_to group_labels_path(@group), title: 'Labels' do
+            %span
+              Labels
+
+        = nav_link(path: 'milestones#index') do
+          = link_to group_milestones_path(@group), title: 'Milestones' do
+            %span
+              Milestones