feat: add manage.py backward compatible script

Part-of: <https://dev.funkwhale.audio/funkwhale/funkwhale/-/merge_requests/2191>

api/manage.py

@@ -0,0 +1,15 @@
+#!/usr/bin/env python3
+
+import warnings
+
+from funkwhale_api.main import main
+
+warnings.warn(
+    DeprecationWarning(
+        "the './manage.py' script has been deprecated, please use the 'funkwhale-manage' "
+        "entrypoint instead (e.g. 'funkwhale-manage migrate')"
+    )
+)
+
+if __name__ == "__main__":
+    SystemExit(main())

changes/changelog.d/manage-py.deprecation

@@ -0,0 +1 @@
+Deprecate the api manage.py script in favor of the funkwhale-manage entrypoint