Fix regression in ef2857667 that caused animated GIFs and PNGs to
generate thumbnails that were larger than 150x150.
Also fix a bug with cropped previews not being generated for animated
GIFs and PNGs.
* 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.
* Add MediaFile abstraction. A MediaFile represents an image or video file.
* Move filetype detection and dimension parsing code from uploads to MediaFile.
