Refactor /api/playlists endpoint

[Noam-Bendelac]

As in #58, only respond with playlists the user has access to. Refine the api response type defined in apiTypes.ts to enumerate the forwarded Spotify response fields and include needed db fields. Use that api response type in the frontend instead of SpotifyApi.PlaylistObjectSimplified[] and do appropriate changes. Decide on a good way to request Spotify playlists (request all? or an array of individual playlist promises?) and consolidate those with db playlists.

[Noam-Bendelac]

Seems like only id, users, name, and image are currently relevant fields. Owner is no longer relevant because the Spotify owner will always be the owner account, and the db doesn't keep track of an owner, just the users array. Need to decide if users array should contain ids or display names; I think I should use display names, after I implement a display name cache in the backend (#38). There will possibly be more relevant fields in the future.