Use StartAudioContext to fix mobile safari audio contexts

package.json

@@ -88,6 +88,7 @@
     "scratch-storage": "^0.2.0",
     "scratch-vm": "latest",
     "selenium-webdriver": "^3.5.0",
+    "startaudiocontext": "1.2.1",
     "style-loader": "^0.18.0",
     "svg-to-image": "1.1.3",
     "svg-url-loader": "2.1.0",

src/lib/audio/shared-audio-context.js

+import StartAudioContext from 'startaudiocontext';
+
 const AUDIO_CONTEXT = new (window.AudioContext || window.webkitAudioContext)();
 
+StartAudioContext(AUDIO_CONTEXT);
+
 /**
  * Wrap browser AudioContext because we shouldn't create more than one
  * @return {AudioContext} The singleton AudioContext