Create deprecation_message partial and use it

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

+.milestone-deprecation-message.prepend-top-default.js-toggle-container
+  = image_tag 'illustrations/milestone_removing-page.svg', class: 'append-right-default'
+  .inline.vertical-align-middle.text-container
+    %strong This page will be removed in a future release.
+    %p.append-bottom-default.prepend-top-5
+      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 js-toggle-button'
+
+    .popup.js-toggle-content.hide.small-triangle
+      .body
+        %ol.instructions-list
+          %li
+            Click any
+            %strong project name
+            in the project list below to navigate to the project milestone.
+          %li
+            Click the
+            %strong Promote
+            button in the top right corner to promote it to a group milestone.
+      .footer= link_to 'Learn more', help_page_url('user/project/milestones', anchor: 'promoting-project-milestones-to-group-milestones'), class: 'btn-link'
+
+    %div= link_to 'Learn more', help_page_url('user/project/milestones', anchor: 'promoting-project-milestones-to-group-milestones'), class: 'btn btn-default'
\ No newline at end of file

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

@@ -30,15 +30,7 @@
         - else
           = link_to 'Reopen Milestone', group_milestone_route(milestone, {state_event: :activate }), method: :put, class: "btn btn-grouped btn-reopen"
 
-.prepend-top-default
-  = image_tag 'illustrations/milestone_removing-page.svg', class: 'append-right-default'
-  .inline.vertical-align-middle
-    %strong This page will be removed in a future release.
-    %p.append-bottom-default.prepend-top-default
-      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', help_page_url('user/project/milestones', anchor: 'promoting-project-milestones-to-group-milestones'), class: 'btn btn-default'
+= render 'shared/milestones/deprecation_message'
 
 .detail-page-description.milestone-detail
   %h2.title