Add activitycontinuation to apple-app-site-association file

[farktronix]

We should add the activitycontinuation property so that handoff from Mac to iOS jumps to the app instead of Safari:

{
  "activitycontinuation": {
    "apps": [    
      "HT7ELV3Q35.com.alecgorge.ios.Listen-to-the-Dead",
      "HT7ELV3Q35.com.alecgorge.Phish-Tracks" ]
  }
}