Decide proper status codes for some endpoints

[Noam-Bendelac]

Living document of what status codes different endpoints will have:

Route	Status code
Errors:
/admin/*, wrong admin key	4031
/api/*, not logged in2	401
/api/playlists/:id[/*], id doesn't exist OR exists but user is not allowed	404
/api/playlists/:id[/*], spotify playlist doesn't exist (if checked)	500
Successes:
POST /api/playlists/:pid/tracks/:tid/chat	201
PUT /api/playlists/:pid/tracks/:tid/removed	200
POST /api/playlists/:id/tracks	201
POST /api/playlists/:id/chat	201

¹ I think it's ok to say "this resource exists but you don't have access" — the code is open source anyway
² No access_token cookie OR access_token not found in userid cache; this assumes the refresh token endpoint will be moved out of /api/