Refactor and Clean-up Viewsets

- Reorder the code inside the methods to fail fast on autorization,
- Replace `int_or_none` with a more generic solution, and move to utils,
- Add `get_values` and move `pk_and_slug` as `get_pk_and_slug` into utils,
- Replace calls to static methos in models local queries,
- Return meaningful status code while creating and updating resources,
- Return `409` when creating or updating a schedule produces a conflict.