Only show nav_link if service desk has been activated for project.

app/views/layouts/nav/sidebar/_project.html.haml

@@ -138,6 +138,7 @@
                 %span
                   Milestones
 
+            - if @project.service_desk_enabled?
               = nav_link(controller: [ :issues, :service_desk ]) do
                 = link_to service_desk_project_issues_url(@project), title: 'Service Desk' do
                   %span Service Desk