Refactors the upload process to pass around temp files, rather than
passing around file paths and directly writing output to the local
filesystem. This way we can pass the storage manager the preview /
sample / original temp files, so it can deal with storage itself.
* Change Download::File#download! to return a temp file.
* Change DanbooruImageResizer and PixivUgoiraConverter to accept/return
temp files instead of file paths.
* Change Upload#generate_resizes to return temp files for previews and samples.
* Change Upload#generate_resizes to generate ugoira .webm samples
synchronously instead of asynchronously.
