refactor scrobblers (#383)

- no need to explicitly pass api key
- move packages up a level
- catch more errors by extended scrobbler interface with IsUserAuthenticated
- move interface to server
- delete scrobbber package, clients implicitly satisfy Scrobble

this also helps with gonic-lastfm-sync

server/ctrladmin/ctrl.go

@@ -22,8 +22,8 @@ import (
 
 	"go.senan.xyz/gonic"
 	"go.senan.xyz/gonic/db"
+	"go.senan.xyz/gonic/lastfm"
 	"go.senan.xyz/gonic/podcasts"
-	"go.senan.xyz/gonic/scrobble/lastfm"
 	"go.senan.xyz/gonic/server/ctrladmin/adminui"
 	"go.senan.xyz/gonic/server/ctrlbase"
 )