Fix milestone sidebar collapse button

app/assets/stylesheets/pages/issuable.scss

@@ -260,7 +260,7 @@
       padding-top: 10px;
     }
 
-    &:not(.issue-boards-sidebar):not([data-signed-in]) {
+    &:not(.issue-boards-sidebar):not([data-signed-in]):not([data-always-show-toggle]) {
       .issuable-sidebar-header {
         display: none;
       }

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

 - affix_offset = local_assigns.fetch(:affix_offset, "50")
 - project = local_assigns[:project]
 
-%aside.right-sidebar.js-right-sidebar{ data: { "offset-top" => affix_offset, "spy" => "affix" }, class: sidebar_gutter_collapsed_class, 'aria-live' => 'polite' }
+%aside.right-sidebar.js-right-sidebar{ data: { "offset-top" => affix_offset, "spy" => "affix", "always-show-toggle" => true }, class: sidebar_gutter_collapsed_class, 'aria-live' => 'polite' }
   .issuable-sidebar.milestone-sidebar
     .block.milestone-progress.issuable-sidebar-header
       %a.gutter-toggle.pull-right.js-sidebar-toggle{ role: "button", href: "#", "aria-label" => "Toggle sidebar" }