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/controllers/uploads_controller.rb

@@ -13,7 +13,7 @@ class UploadsController < ApplicationController
         "User-Agent" => "#{Danbooru.config.safe_app_name}/#{Danbooru.config.version}"
       }
       Downloads::RewriteStrategies::Base.strategies.each do |strategy|
-        @normalized_url, headers = strategy.new.rewrite(@normalized_url, headers)
+        @normalized_url, headers = strategy.new(@normalized_url).rewrite(@normalized_url, headers)
       end
 
       @post = Post.find_by_source(@normalized_url)