flash files can be quite big (the biggest on danbooru.donmai.us being
68.6MB atm). Reading it and applying complex transformations twice seems
unnecessary.
MediaFile#dimensions is called twice - in #width and in #height but
it only works on the first call because the file is read to the end and
consumed the first time so when #read is called the second time it only
returns the empty string
* 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.
* Use libvips instead of ruby-imagespec for reading dimensions of jpeg, png, and gif files.
* Copy the code for reading the dimensions of flash files from ruby-imagespec.
Fixes an incompatibility between ruby-imagespec and the rubocop gem that
prevented us from including rubocop in the Gemfile.
* Add MediaFile abstraction. A MediaFile represents an image or video file.
* Move filetype detection and dimension parsing code from uploads to MediaFile.
