show gear image when no thumbnail URL present

Okay, surprise, the bug was unrelated to Camo config (though I'm glad I cleaned
that up anyway :P). We now, at a low level, serve a placeholder image for item
thumbnail URL if, for some reason, we don't have a good thumbnail URL on hand.

app/models/item.rb

@@ -328,7 +328,13 @@ class Item < ActiveRecord::Base
   end
 
   def thumbnail
-    @thumbnail ||= Image.from_insecure_url(thumbnail_url)
+    if thumbnail_url.present?
+      url = thumbnail_url
+    else
+      url = ActionController::Base.helpers.asset_path(
+        "broken_item_thumbnail.gif")
+    end
+    @thumbnail ||= Image.from_insecure_url(url)
   end
 
   def as_json(options={})