Move "add cluster" button partial to match CE

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

@@ -10,7 +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 'projects/ee/clusters/buttons', project: @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" }

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

+.nav-controls
+  - if clusterable.feature_available?(:multiple_clusters)
+    = link_to s_("ClusterIntegration|Add Kubernetes cluster"), clusterable.new_path, class: "btn btn-success btn-add-cluster has-tooltip js-add-cluster"
+  - else
+    %span.btn.btn-add-cluster.disabled.js-add-cluster
+      = s_("ClusterIntegration|Add Kubernetes cluster")

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

-.nav-controls
-  - if project.feature_available?(:multiple_clusters)
-    = link_to s_("ClusterIntegration|Add Kubernetes cluster"), new_project_cluster_path(project), class: "btn btn-success btn-add-cluster has-tooltip js-add-cluster"
-  - else
-    = link_to s_("ClusterIntegration|Add Kubernetes cluster"), new_project_cluster_path(project), class: "btn btn-success btn-add-cluster disabled has-tooltip js-add-cluster", title: s_("ClusterIntegration|Multiple Kubernetes clusters are available in GitLab Enterprise Edition Premium and Ultimate")