Allow ugoira to be uploaded directly from the zip URL.

This refactors things such that

* Fetching the ugoira frame data happens at the end of rewrite, not
  rewrite_html_pages. This way uploading zip URLs should work.
* The source data is cached in an instance variable in case it
  was indeed fetched during rewrite_html_pages. This way it doesn't get
  fetched a second time.

app/logical/downloads/rewrite_strategies/base.rb

@@ -1,6 +1,10 @@
 module Downloads
   module RewriteStrategies
     class Base
+      def initialize(url = nil)
+        @url = url
+      end
+
       def self.strategies
         [Pixiv, NicoSeiga, Twitpic, DeviantArt, Tumblr, Moebooru]
       end