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/file.rb

@@ -29,7 +29,7 @@ module Downloads
 
     def before_download(url, headers, datums)
       RewriteStrategies::Base.strategies.each do |strategy|
-        url, headers, datums = strategy.new.rewrite(url, headers, datums)
+        url, headers, datums = strategy.new(url).rewrite(url, headers, datums)
       end
 
       return [url, headers, datums]