Ingest user’s Spotify library

[maeve]

On second thought, we're probably going to use a combination of Spotify's seeded recommendations and top tracks/artists for a particular user to back this implementation so that we don't need to know the entire contents of a user's libary.

See:

• https://developer.spotify.com/documentation/web-api/reference/browse/get-recommendations/
• https://developer.spotify.com/documentation/web-api/reference/personalization/get-users-top-artists-and-tracks/

[maeve]

Reopening as a non-MVP enhancement. @sleverbor would very much like to be able to use his own library as input to our rules engine, in order to apply rules based on playback timestamps from last.fm (see #9).

I don't think this is incompatible with the rest of mixtape. All of the functionality for authenticating, rule CRUD, applying rules to an input set of tracks, creating/refreshing playlists, etc. would still be the same. The big difference is whether the initial set of tracks comes from Spotify's recommendation engine (see #4) vs the user's own library. I can also see a use case where someone wants to start with another playlist as the input.