FEATURE: generate (avatar) thumbnails in a background task

FIX: keep the "uploading..." indicator until the server replies via the MessageBus
FIX: text was disapearing when uploading an avatar

PERF: always use a region for S3 (defaults to 'us-east-1')
FEATURE: ApplyCDN middleware when using S3
FIX: use the same pattern to store files on S3 and locally
PERF: keep a local cache of uploads when generating thumbnails
FEATURE: migrate_to_s3 rake task

lib/middleware/apply_cdn.rb

@@ -0,0 +1,24 @@
+module Middleware
+
+  class ApplyCDN
+
+    def initialize(app, settings={})
+      @app = app
+    end
+
+    def call(env)
+      status, headers, response = @app.call(env)
+
+      if Discourse.asset_host.present? &&
+         Discourse.store.external? &&
+        (headers["Content-Type"].start_with?("text/") ||
+         headers["Content-Type"].start_with?("application/json"))
+        response.body = response.body.gsub(Discourse.store.absolute_base_url, Discourse.asset_host)
+      end
+
+      [status, headers, response]
+    end
+
+  end
+
+end