Cleans cluster views a bit

- Moves logic into ClustersHelp
- Add CSS to ensure compatibility with EE view.

app/helpers/clusters_helper.rb

@@ -36,4 +36,8 @@ module ClustersHelper
       render 'clusters/clusters/gcp_signup_offer_banner'
     end
   end
+
+  def display_clusters_callout?(clusters, clusterable)
+    clusters.length > clusterable.clusters.length
+  end
 end

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

 -# This partial is overridden in EE
 .nav-controls
   - if clusterable.can_create_cluster? && clusterable.clusters.empty?
-    = link_to s_('ClusterIntegration|Add Kubernetes cluster'), clusterable.new_path, class: 'btn btn-success'
+    = link_to s_('ClusterIntegration|Add Kubernetes cluster'), clusterable.new_path, class: 'btn btn-success js-add-cluster'
   - else
     %span.btn.btn-add-cluster.disabled.js-add-cluster
       = s_("ClusterIntegration|Add Kubernetes cluster")

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

@@ -12,7 +12,7 @@
         = s_("ClusterIntegration|Kubernetes clusters can be used to deploy applications and to provide Review Apps for this project")
       = render 'clusters/clusters/buttons'
 
-    - if @clusters.length > clusterable.clusters.length
+    - if display_clusters_callout?(@clusters, clusterable)
       .bs-callout.bs-callout-info
         = s_("ClusterIntegration|Clusters are utilized by selecting the nearest ancestor with a matching environment scope. For example, project clusters will override group clusters.")
         %strong