* 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.