Add get_or_create_playlist and follow_playlist methods to RemoteAPI and related changes

[geo-martino]

Added

• :py:meth:.RemoteAPI.get_or_create_playlist method for only creating a playlist when it doesn't
  already exist by name. Gets the existing playlist otherwise
• Added :py:meth:.MusifyCollection.outer_difference method to cover the logic previously handled
  by the mislabelled :py:meth:.MusifyCollection.outer_difference method
• :py:class:.RemoteDataWrangler and its implementations now handle URL objects from the yarl package
• :py:meth:.RemoteAPI.follow_playlist method

Changed

• :py:class:.RemoteItemChecker now uses the new :py:meth:.RemoteAPI.get_or_create_playlist method
  when creating playlists to avoid creating many duplicate playlists which could have lead to playlist
  creation explosion in repeated uses. The processor also accounts for any items that may have existed
  in the playlist before it was run and discounts them from any matches.
• :py:class:.RemoteItemChecker also uses the new :py:meth:.RemoteAPI.follow_playlist method
  when creating playlists to ensure that a user is following the playlists it creates to avoid 'ghost playlist' issue.
• :py:meth:.SpotifyAPI.create_playlist now returns the full response rather than just the URL of the playlist.

Fixed

• Fixed logic in :py:meth:.MusifyCollection.intersection and :py:meth:.MusifyCollection.difference