* Move image thumbnail generation code to MediaFile::Image.
* Move video thumbnail generation code to MediaFile::Video.
* Move ugoira->webm conversion code to MediaFile::Ugoira.
This separates thumbnail generation from the upload process so that it's
possible to generate thumbnails outside of uploads.
* 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.
