Add CSV download for User

app/views/admin/users/index.html.haml

@@ -2,6 +2,9 @@
 
 - content_for :actionbar do
   = link_to t('.new_user'), new_admin_user_path, class: 'btn btn-primary'
+  = link_to url_for(search: params[:q], format: :csv), class: 'btn' do
+    = glyph :download
+    CSV
 
 - content_for :sidebar do
   %p= t('.first_paragraph', url: link_to(t('.new_users'), new_admin_user_path)).html_safe