Add audio-only option to transcoders and player

This patch adds an audio-only option to PeerTube by means of a new transcoding configuration which creates mp4 files which only contain an audio stream. This new transcoder has a resolution of '0' and is presented in the preferences and in the player resolution menu as 'Audio-only' (localised). When playing such streams the player shows the file thumbnail as background and disables controls autohide.

Audio-only files can be shared and streamed just like any other file. They can be downloaded as well, the resulting file will be an mp4 container with a single audio stream.

This patch is a proof of concept to show the feasibility of 'true' audio-only support. There are better ways of doing this which also enable multiple audio streams for a given video stream (e.g. DASH) but as this would entail a fundamental change in the way PeerTube works it is a bridge too far for a simple proof of concept.

shared/extra-utils/server/config.ts

@@ -111,6 +111,7 @@ function updateCustomSubConfig (url: string, token: string, newConfig: DeepParti
       allowAudioFiles: true,
       threads: 1,
       resolutions: {
+        '0p': false,
         '240p': false,
         '360p': true,
         '480p': true,