Use JPEG for client posting instead of PNG.

This leaves the glitch -> PNG step intact, so ffmpeg is still dealing with PNGs. This also fixes a number of bugs in the media conversion process, and removes unnecessary setImmediate calls.

Saves a ton of upload for clients: ~950k per recording with PNG to ~130k with JPEG. We could probably bump the quality up even more if desired, but this seems relatively similar to the PNG results.