Separate HLS audio and video streams

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

server/core/initializers/migrations/0865-video-file-streams.ts

@@ -0,0 +1,53 @@
+import * as Sequelize from 'sequelize'
+
+async function up (utils: {
+  transaction: Sequelize.Transaction
+  queryInterface: Sequelize.QueryInterface
+  sequelize: Sequelize.Sequelize
+}): Promise<void> {
+  const { transaction } = utils
+
+  {
+    await utils.queryInterface.addColumn('videoFile', 'formatFlags', {
+      type: Sequelize.INTEGER,
+      defaultValue: 2, // fragmented
+      allowNull: false
+    }, { transaction })
+
+    // Web videos
+    const query = 'UPDATE "videoFile" SET "formatFlags" = 1 WHERE "videoId" IS NOT NULL'
+    await utils.sequelize.query(query, { transaction })
+
+    await utils.queryInterface.changeColumn('videoFile', 'formatFlags', {
+      type: Sequelize.INTEGER,
+      defaultValue: null,
+      allowNull: false
+    }, { transaction })
+  }
+
+  {
+    await utils.queryInterface.addColumn('videoFile', 'streams', {
+      type: Sequelize.INTEGER,
+      defaultValue: 3, // audio + video
+      allowNull: false
+    }, { transaction })
+
+    // Case where there is only an audio stream
+    const query = 'UPDATE "videoFile" SET "streams" = 2 WHERE "resolution" = 0'
+    await utils.sequelize.query(query, { transaction })
+
+    await utils.queryInterface.changeColumn('videoFile', 'streams', {
+      type: Sequelize.INTEGER,
+      defaultValue: null,
+      allowNull: false
+    }, { transaction })
+  }
+}
+
+function down (options) {
+  throw new Error('Not implemented.')
+}
+
+export {
+  down, up
+}