Fix too loose strong parameters in the admin interface

There are some attributes that we don't use and/or that should
not be changeable, even by admins.

app/controllers/admin/sponsors_controller.rb

@@ -52,7 +52,7 @@ module Admin
     private
 
     def sponsor_params
-      params.require(:sponsor).permit(:name, :description, :website_url, :picture, :picture_cache, :sponsorship_level_id, :conference_id)
+      params.require(:sponsor).permit(:name, :description, :website_url, :picture, :picture_cache, :sponsorship_level_id)
     end
 
     def sponsorship_level_required