Implement currentPlaylistService, queueService, and playlistService

@deini @malucky 

The playlistService has not been implemented fully because we'll need to work on the userService first, but we can at least add to it the repo at this point. 
Should we return `null` here so whoever calls this function always expect something to return and do whatever it has to do? For example maybe later we need the player to know when a playlist ends or something so it can just check for null? /discuss