Possibly move user display name fetch to backend

[Noam-Bendelac]

Make the /api/playlists/:playlistId/ backend endpoint do more (fetching added_by user data for display name), and have the frontend be simpler? Considering the question of small atomic endpoints versus more complicated endpoints that provide all the data the user needs: I'm already going to bundle the Spotify request with the database, so it might make sense to add this functionality too.

[Noam-Bendelac]

Need to add a cache of user id -> display name with TTL in the backend for this.