feat: move storage path setting to configuration file

- change default storage path to /srv/libretime
- remove music dirs table
- use /tmp for testing storage
- storage dir should always have a trailing slash

api/libretime_api/storage/models/__init__.py

@@ -1,4 +1,3 @@
 from .cloud_file import CloudFile
 from .file import File
-from .storage import MusicDir
 from .track_type import TrackType