[bugfix]: Fix add to playlist success message

The prior code used `form.addToPlaylist`, not `.success`. Also fixes English pluralization and
uses the correct `entity.track` as opposed to `entity.song` for other languages (I am not sure
if the en syntax could be applied to other languages, so I will just leave pluralization as-is
for now).

src/i18n/locales/en.json

@@ -216,7 +216,7 @@
         "addToPlaylist": {
             "input_playlists": "$t(entity.playlist_other)",
             "input_skipDuplicates": "skip duplicates",
-            "success": "added {{message}} $t(entity.song_other) to {{numOfPlaylists}} $t(entity.playlist_other)",
+            "success": "added $t(entity.trackWithCount, {\"count\": {{message}} }) to $t(entity.playlistWithCount, {\"count\": {{numOfPlaylists}} })",
             "title": "add to $t(entity.playlist_one)"
         },
         "createPlaylist": {