Create disabled add cluster button partial to match EE

app/views/clusters/clusters/_buttons.html.haml

+-# This partial is overridden in EE
+.nav-controls
+  %span.btn.btn-add-cluster.disabled.js-add-cluster
+    = s_("ClusterIntegration|Add Kubernetes cluster")

app/views/clusters/clusters/index.html.haml

@@ -10,6 +10,7 @@
     .top-area.adjust
       .nav-text
         = s_("ClusterIntegration|Kubernetes clusters can be used to deploy applications and to provide Review Apps for this project")
+      = render 'clusters/clusters/buttons'
     .ci-table.js-clusters-list
       .gl-responsive-table-row.table-row-header{ role: "row" }
         .table-section.section-30{ role: "rowheader" }