GIF to video

[jace]

Separate from #55, which deals with resizing while retaining the GIF image format, we need the ability to render a GIF as a video in MP4 or WebM formats.

1. We need to use an external convertor as PIL/Pillow is clearly not capable. This ticket will depend on a reliable sandboxed external convertor framework being available.

2. Videos cannot be placed under an <img/> tag and it's unclear if they'll work under <picture><source/></picture> (test pending). Imgee users cannot blindly paste URLs into img tags. Imgee must provide additional API endpoints (and corresponding UI) that helps users construct valid HTML.

[jace]

Imgur's GIFV format announcement offers a hint on how to think about this. They use a video tag with MP4 and WebM options.