Remove all `Promise` in favor of `Observable`

[Brad Kovach]

In an attempt to finish this project quickly, I used Promises in several key areas of the application. Although they work just fine, they defy Angular convention.

• playlist-detail-component

  • ngOnInit: Lines 34-47
  • loadPlaylist: Lines 50-68

• playlist-list-component.ts

  • ngOnInit: Lines 36-56
  • autoRemove: Lines 100-166

• spotify.service.ts

  • removeRKelly: 262-317