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 Now we're rolling our own with an outdated binary.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 (Ffmpeg encoding configuration structure needs work #216)
• No type support and comes with no validation. (Easy to pass invalid FFMpeg arguments in configuration #36)
• If the subprocess errors out, it still continues on like the encode has finished successfully (FFMpeg not failing properly #37)