Add deprecation warning to shared/milestones/top

app/views/shared/milestones/_top.html.haml

@@ -30,6 +30,16 @@
         - else
           = link_to 'Reopen Milestone', group_milestone_route(milestone, {state_event: :activate }), method: :put, class: "btn btn-grouped btn-reopen"
 
+.deprecation-warning.prepend-top-default
+  = image_tag 'illustrations/milestone_removing-page.svg'
+  .inline
+    %strong.append-top-default This page will be removed in a future release.
+    %p
+      Use group milestones to manage issues from multiple projects in the same milestone.
+      %br
+      = button_tag 'Promote these project milestones into a group milestone.', class: 'btn-link'
+    %div= link_to 'Learn more', class: 'btn btn-default'
+
 .detail-page-description.milestone-detail
   %h2.title
     = markdown_field(milestone, :title)