Merge branch '10428-cluster-sidebar' into 'master'

Removes EE differences for clusters/_sidebar.html.haml

Closes gitlab-ee#10428

See merge request gitlab-org/gitlab-ce!26271

app/views/clusters/clusters/_sidebar.html.haml

@@ -4,3 +4,5 @@
   = clusterable.sidebar_text
 %p
   = clusterable.learn_more_link
+
+= render_if_exists 'clusters/multiple_clusters_message'