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.