Code refactoring

- Code refactoring mainly for API
- Delete a useless option in `setting.py`
- Change some constants in Link Play mode

latest version/api/__init__.py

@@ -0,0 +1,10 @@
+from flask import Blueprint
+
+from . import users
+from . import songs
+from . import token
+
+bp = Blueprint('api', __name__, url_prefix='/api/v1')
+bp.register_blueprint(users.bp)
+bp.register_blueprint(songs.bp)
+bp.register_blueprint(token.bp)