* Make it so replacing a post doesn't generate a dummy upload as a side effect.
* Make it so you can't replace a post with itself (the post should be regenerated instead).
* Refactor uploads and replacements to save the ugoira frame data when
the MediaAsset is created, not when the post is created. This way it's
possible to view the ugoira before the post is created.
* Make `download_file!` in the Pixiv source strategy return a MediaFile
with the ugoira frame data already attached to it, instead of returning it
in the `data` field then passing it around separately in the `context`
field of the upload.
Add methods to MediaFile to calculate the duration, frame count, and
frame rate of animated GIFs, PNGs, Ugoiras, and videos.
Some considerations:
* It's possible to have a GIF or PNG that's technically animated but
just has one frame. These are treated as non-animated images.
* It's possible to have an animated GIF that has an unspecified
frame rate. In this case we assume the frame rate is 10 FPS; this is
browser dependent and may not be correct.
* Animated GIFs, PNGs, and Ugoiras all support variable frame rates.
Technically, each frame has a separate delay, and the delays can be
different frame-to-frame. We report only the average frame rate.
* Getting the duration of an APNG is surprisingly hard. Most tools don't
have good support for APNGs since it's a rare and non-standardized
format. The best we can do is get the frame count using ExifTool and the
frame rate using ffprobe, then calculate the duration from that.
* 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.
