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

scrobble/scrobble.go

@@ -1,11 +0,0 @@
-package scrobble
-
-import (
-	"time"
-
-	"go.senan.xyz/gonic/db"
-)
-
-type Scrobbler interface {
-	Scrobble(user *db.User, track *db.Track, stamp time.Time, submission bool) error
-}