Fix playlist and cover art download

[stephanlensky]

This fixes a few problems caused by the latest Spotify API updates.

• The old track JSON endpoint no longer works (which was needed to retrieve cover art), so Spotipy is now used instead. Note that this fix has only been applied to the --large-cover-art option (since that's what I use). Without this option, cover art remains broken.
• Playlist loading now correctly makes use of pagination in the API. Before, only a single request was made, which caps the loaded playlist size at 100.
• Also, for some reason, playlist emptying was still on by default (regardless of the --remove-from-playlist option). I commented the code causing this.

[stephanlensky]

Actually, my bad, I see a lot of these issues have been fixed already in another branch. Feel free to close this PR. Worth noting though that I think that the playlist loading is broken in that branch as well.