Historical records: set the change reason explicitly on the instance

[stsewd]

The implementation of django-simple-history's update_change_reason
is very brittle, as it queries for a previous historical record
that matches the attributes of the instance to update the change_reason,
which could end up updating the wrong record, or not finding it.

https://github.com/jazzband/django-simple-history/blob/b8e43df626a03ce4817beaf6a20995b40a209f37/simple_history/utils.py#L27

I saw this mostly happening when:

• Saving a team without changing anything (update_change_reason won't
  find the record for some reason)
• Deleting a team (update_change_reason will override the previous
  record instead)

But simple history also mentions that you can set the change reason
explicitly by setting the _change_reason attribute,
which is more safer as it sets the change_reason at the creation time of
the record.

https://django-simple-history.readthedocs.io/en/latest/historical_model.html#change-reason

I have replaced almost all usages of safe_update_change_reason, except for the ones on apiv3

readthedocs.org/readthedocs/api/v3/mixins.py

Lines 35 to 43 in dbe27b3

	def perform_create(self, serializer):
	obj = serializer.save()
	safe_update_change_reason(obj, self.get_change_reason())
	return obj
	def perform_update(self, serializer):
	obj = serializer.save()
	safe_update_change_reason(obj, self.get_change_reason())
	return obj

as we don't have access to the object before it's updated/created.

Also, maybe we can have a set_change_reason function instead, so it's less error-prone to get the attribute name wrong.

[stsewd]

I just made this a function and was able to use in the update method of apiv3, create is still using the old method since creation happens inside the method and they use .create(**kwargs) instead of creating an instance first.