Playlist API support

[Toastbrot236]

Some example endpoint routes:

GET /playlists/id/{id}
POST /playlists/new or just /playlists to create a new playlist
PATCH /playlists/id/{id} to update a playlist's metadata
DELETE /playlists/id/{id}
POST /playlists/id/{id}/add/level/{id}
POST /playlists/id/{id}/add/playlist/{id}
POST /playlists/id/{id}/remove/level/{id}
POST /playlists/id/{id}/remove/playlist/{id}

Incase #952 gets implemented, also:

GET /playlists
GET /playlists/{route}

Incase playlist categories get implemented, we could also implement getting playlists containing a playlist/level or levels/playlists in a playlist using hidden categories like this:

/playlists/inPlaylist
/playlists/containingPlaylist
/playlists/containingLevel
/levels/inPlaylist

where the client would have to specify either the target playlist's ID with a p or playlist query param, or the target level's ID with a l or level query param. Maybe leave the "Level" or "Playlist" out of the category routes since the query params already state whether it's for a level or playlist? Maybe the user could also optionally include the username or uuid of a user with a u or user query param for these categories to only return the levels/playlists by that user, to also recreate that LBP1 feature?