* Fix corrupted image detection. We were shelling out to vips and trying
to grep for error messages, but the error message for jpeg files changed.
Now we load the file in ruby vips, which raises an error on failure.
* Don't attempt to redownload corrupted images. If a download completes
without any errors yet the downloaded file is corrupt, then something is
wrong at the source and redownloading is unlikely to help. Let the
upload fail and the user retry if necessary.
* Validate that all uploads are uncorrupted, including files uploaded
from a computer, not just files uploaded from a source.
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.
