Add support to Wagtail media for transcoding of videos to different formats using FFMPEG`
Extend Wagtail Media with the ability to transcode videos to different formats. (See https://github.com/takeflight/wagtailvideos for how this might be implemented.)
New features:
- Add "formats", a model which can be used to manage different versions of the video, similar to "renditions" for images
- Allow a default set of formats to be created on initial video upload (specified in the application settings)
- Allow the platform to create different versions of the file, some of which might be optimized for mobile and others for desktop
- If no poster is provided at upload, extract an image from the video during transcoding to use as the poster