modularize src

+ merge main.py and manage.py
+ move start logic to swingmusic/__main__.py
+ add a run.py on the project root

swingmusic/migrations/base.py

@@ -0,0 +1,12 @@
+class Migration:
+    """
+    Base migration class.
+    """
+    enabled: bool = True
+
+    @staticmethod
+    def migrate():
+        """
+        Code to run when migrating, override this method.
+        """
+        pass