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.
Only fetch ugoira data for ugoira URLs.

Don't fetch ugoira data for http://i1.pixiv.net/img35/img/kinokoyarou/46165361.png URLs.
Merge pull request #2271 from evazion/ugoira

Allow ugoira to be uploaded directly from the zip URL.