enable thumbnailing on S3

- added url to optimized image model
- refactored s3_store & local_store

app/models/topic_link.rb

@@ -103,8 +103,8 @@ class TopicLink < ActiveRecord::Base
           topic_id = nil
           post_number = nil
 
-          if Upload.has_been_uploaded?(url)
-            internal = !Upload.is_on_s3?(url)
+          if Discourse.store.has_been_uploaded?(url)
+            internal = Discourse.store.internal?
           elsif parsed.host == Discourse.current_hostname || !parsed.host
             internal = true