Add Vue issuable sidebar app mount-point

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

@@ -5,7 +5,11 @@
 - signed_in = !!issuable_sidebar.dig(:current_user, :id)
 - can_edit_issuable = issuable_sidebar.dig(:current_user, :can_edit)
 
-%aside.right-sidebar.js-right-sidebar.js-issuable-sidebar{ data: { signed: { in: signed_in } }, class: sidebar_gutter_collapsed_class, 'aria-live' => 'polite' }
+- if Feature.enabled?(:vue_issuable_sidebar, @project.group)
+  %aside#js-vue-issuable-sidebar{ data: { signed_in: signed_in,
+    sidebar_status_class: sidebar_gutter_collapsed_class } }
+- else
+  %aside.right-sidebar.js-right-sidebar.js-issuable-sidebar{ data: { signed: { in: signed_in } }, class: sidebar_gutter_collapsed_class, 'aria-live' => 'polite' }
     .issuable-sidebar
       .block.issuable-sidebar-header
         - if signed_in