Fix new audio player features not working on Safari (#14465)

Fixes #14462

app/javascript/mastodon/features/audio/index.js

@@ -269,6 +269,7 @@ class Audio extends React.PureComponent {
   }
 
   _initAudioContext () {
+    const AudioContext = window.AudioContext || window.webkitAudioContext;
     const context      = new AudioContext();
     const source       = context.createMediaElementSource(this.audio);