Simplify FFMPEG video codec settings

[chrisbenincasa]

Instead of allowing the user to specify an encoder directly, we should boil this option down to a couple "families" of codecs, based on available hardware. This will allow the backend to decide the best decoder/encoder for the job and simplify the onboarding, too. This is especially important with the FFMPEG pipeline rewrite happening in #389.

The options should be: "no hardware acceleration" (or "on CPU", or "software"), "Nvidia", "Intel QuickSync", and "VAAPI" (or "Video Acceleration API")

This change will affect both our settings database file, ffmpeg pipeline, as well as the frontend, so it is a rather large change.