Consider an FFmpeg library

[in03]

We're rolling our own atm and it comes with some downsides.

• I forked better-ffmpeg-progress and reworked it for Celery progress. Now we're rolling our own with an outdated binary.
• Argument passing is pretty clunky (#216)
• No type support and comes with no validation. (#36)
• If the subprocess errors out, it still continues on like the encode has finished successfully (#37)