Fragmented MP4 with hls.js

[knackebrot]

Fragmented MP4 significantly reduces the muxing overhead of HLS fragments, so I think it would be worthwhile to try to make it work on browsers without native HLS support. Starting playback from the beginning works fine, but seeking is problematic. When seeking beyond buffered range (when a new transcoding job is started), the new initialization mp4 file doesn't get downloaded and the playback breaks, though new fragments keep getting downloaded as if it worked fine. Needs investigation whether we can configure hls.js to work, or if it would require a fork to handle our arguably weird use case.