Fix issue on many to many fields in all forms

[susilnem]

Issue description

For example: Currently, the fields that are M2M in the DREF forms are creating a new instance in every form update. Due to this, the translation for the required fields are being triggered in every update. This could be one of the reasons why the translation costs are currently very high since these fields are using the translation API a lot.

Solution

• While updating the DREF forms, a new instance should not be created in every form update if the ID is passed from the frontend. If the ID is NOT passed, a new instance should be created.
• Update the CUD in Serializers for M2M fields

[szabozoltan69]

E.g. planned_interventions could be fixed like this:

def update(self, instance, validated_data):
    planned_interventions_data = validated_data.pop('planned_interventions', None)
    # ...existing code...

    if planned_interventions_data is not None:
        intervention_ids = []
        for intervention in planned_interventions_data:
            intervention_id = intervention.get('id')
            if intervention_id:
                # Update existing one
                obj = PlannedIntervention.objects.get(id=intervention_id)
                for attr, value in intervention.items():
                    setattr(obj, attr, value)
                obj.save()
                intervention_ids.append(obj.id)
            else:
                # Create a new one
                obj = PlannedIntervention.objects.create(**intervention)
                intervention_ids.append(obj.id)
        instance.planned_interventions.set(intervention_ids)

    # ...existing code...
    instance.save()
    return instance