add milestone reference clipboard button to sidebar

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

@@ -3,7 +3,20 @@
 %aside.right-sidebar.js-right-sidebar{ data: { "offset-top" => affix_offset, "spy" => "affix" }, class: sidebar_gutter_collapsed_class, 'aria-live' => 'polite' }
   .issuable-sidebar
     .block.issuable-sidebar-header
-      %span.issuable-header-text.hide-collapsed.pull-left
-        Milestone #{milestone.try(:to_reference) || milestone.title}
       %a.gutter-toggle.pull-right.js-sidebar-toggle{ role: "button", href: "#", "aria-label" => "Toggle sidebar" }
         = sidebar_gutter_toggle_icon
+
+      %span.issuable-header-text.hide-collapsed
+        Milestone #{milestone.try(:to_reference) || milestone.title}
+
+    - milestone_ref = milestone.try(:to_reference, full: true)
+    - if milestone_ref.present?
+      .block
+        .sidebar-collapsed-icon.dont-change-state
+          = clipboard_button(clipboard_text: milestone_ref, title: "Copy reference to clipboard", placement: "left")
+        .cross-project-reference.hide-collapsed
+          %span
+            Reference:
+            %cite{ title: milestone_ref }
+              = milestone_ref
+          = clipboard_button(clipboard_text: milestone_ref, title: "Copy reference to clipboard", placement: "left")