Oops, fix redirect to item thumbnail

This `.gif` format is used in the items list "export to petpage"
feature, as the image URL for items whose URLs are known to contain
blocked words that prevent them from being used in petpages.

But when doing some Rails upgrade long ago, we didn't notice the new
security feature that blocks redirects to other sites without a special
flag being set. It was triggering 500 errors, oops.

Now, we set the flag!

app/controllers/items_controller.rb

@@ -86,7 +86,7 @@ class ItemsController < ApplicationController
 
       format.gif do
         expires_in 1.month
-        redirect_to @item.thumbnail_url
+        redirect_to @item.thumbnail_url, allow_other_host: true
       end
     end
   end