Render related content form just to signed in users

It doesn't make much sense to render a hidden form to users who will not
be able to use it.

app/views/relationable/_related_content.html.erb

@@ -11,7 +11,9 @@
     <% end %>
   </div>
 
+  <% if current_user %>
     <%= render "relationable/form", relationable: relationable %>
+  <% end %>
 
   <% if @related_contents.present? %>
     <%= render "relationable/related_list", relationable: relationable %>