Implement add song to playlist screen

[mrf7]

Assume the user has a selected playlist (from previous screen or hardcoded for testing) and provide a simple UI with a text box and a button

• When the user pastes the link to a song on spotify in the text box and hits the button, the app should attempt to add the song to the selected playlist.
• When the request completes, display a simple success/failure indication (i.e. toast/snackbar)
• Should be able to parse the link out of the standard spotify share text
  "Here's a song for you...."