Ignore migrations
Remove storage from LastfmUser and SpotifyUser
Access starred tracks via the User, not directly from the SpotifyUser

- SpotifyUser and LastfmUser take a ConnectionHelper instead of a User
- User proxies starred tracks to SpotifyUser
Move all get_model repository methods into a separate module

Solves circular dependency issues when repositories needed to
include eachother.
Store user's Spotify tracks in the db

- Create the SpotifyPlaylistTrack that includes date added by the user

- Don't instantiate a User with starred tracks, always get them from the
  SpotifyUser or the spotify_playlist_track_repository

- Return SpotifyPlaylistTrack from User#starred_tracks instead of a dict