Updates in song model.

Was getting this error while adding the lyrics. Issue and patch mentioned here Automattic/mongoose#5574 (comment)

```bundle.js:6882 Uncaught (in promise) Error: GraphQL error: Unknown modifier: $pushAll
    at new ApolloError (bundle.js:6885)
    at bundle.js:15869```

server/models/song.js

@@ -11,7 +11,7 @@ const SongSchema = new Schema({
     type: Schema.Types.ObjectId,
     ref: 'lyric'
   }]
-});
+}, { usePushEach: true });
 
 SongSchema.statics.addLyric = function(id, content) {
   const Lyric = mongoose.model('lyric');