Add tooltip to subscribe btn

app/views/projects/issues/index.html.haml

@@ -19,7 +19,7 @@
       = render 'shared/issuable/nav', type: :issues
       .nav-controls
         - if current_user
-          = link_to url_for(params.merge(format: :atom, private_token: current_user.private_token)), class: 'btn append-right-10', title: 'Subscribe' do
+          = link_to url_for(params.merge(format: :atom, private_token: current_user.private_token)), class: 'btn append-right-10 has-tooltip', title: 'Subscribe' do
             = icon('rss')
         - if can? current_user, :create_issue, @project
           = link_to new_namespace_project_issue_path(@project.namespace,