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/models/videos/video-resolution.enum.ts

@@ -1,6 +1,7 @@
 import { VideoTranscodingFPS } from './video-transcoding-fps.model'
 
 export enum VideoResolution {
+  H_NOVIDEO = 0,
   H_240P = 240,
   H_360P = 360,
   H_480P = 480,
@@ -18,6 +19,10 @@ export enum VideoResolution {
  */
 function getBaseBitrate (resolution: VideoResolution) {
   switch (resolution) {
+    case VideoResolution.H_NOVIDEO:
+      // audio-only
+      return 64 * 1000
+
     case VideoResolution.H_240P:
       // quality according to Google Live Encoder: 300 - 700 Kbps
       // Quality according to YouTube Video Info: 186 Kbps