Automation Rules: simplify ordering

[stsewd]

I'm trying to re-use this for redirects, and found a couple of things that we can improve.

• New items would be inserted first instead of at the end, if a user is adding a new item, they may want to have it priority over existing ones. Inserting the item at the end was mostly done because it was "easier" to implement.

• Instead of explicitly using the manager to add a new rule, just override the save() method. This allows us to:

  • Insert a new rule with any priority
  • Change the priority of a rule by just saving it with the new priority.

  With this, we no longer need to override forms, serializers, etc, saving the model will take care of putting the rule in the right place.

---

📚 Documentation previews 📚

• User's documentation (docs): https://docs--10896.org.readthedocs.build/en/10896/

• Developer's documentation (dev): https://dev--10896.org.readthedocs.build/en/10896/

[humitos]

The new approach looks better to me.

I think the _change_priority() is unnecessary complex and can be re-written in a lot more simple way. I proposed a way that's lot easier to understand and maintain.