Allows:
* The HLS player to propose an "Audio only" resolution
* The live to output an "Audio only" resolution
* The live to ingest and output an "Audio only" stream
This feature is under a config for VOD videos and is enabled by default for lives
In the future we can imagine:
* To propose multiple audio streams for a specific video
* To ingest an audio only VOD and just output an audio only "video"
(the player would play the audio file and PeerTube would not
generate additional resolutions)
This commit introduce a new way to download videos:
* Add "/download/videos/generate/:videoId" endpoint where PeerTube can
mux an audio only and a video only file to a mp4 container
* The download client modal introduces a new default panel where the
user can choose resolutions it wants to download
