Add audio file in the `Song` model

[Neraste]

Currently, the feeder detects the instrumental track by either an extra audio file or an extra audio track, but the server cannot store this distinction. A new field filename_instrumental should be added to Song to keep track of it. 3 possibilities would be allowed:

• has_instrumental=False and filename_instrumental=None: no instrumental track;
• has_instrumental=True and filename_instrumental=None: instrumental track in the video file;
• has_instrumental=True and filename_instrumental="file.ext": instrumental track in the audio file.

This new field should be passed to the player only.