Simplify posts property on Category model

[stuartmaxwell]

The following code in category.py, line 116:

    @property
    def posts(self) -> models.QuerySet:
        """Return only published posts."""
        return self._posts.filter(
            status="published",
            date__lte=timezone.now(),
        )

Can be changed to:

    @property
    def posts(self) -> models.QuerySet:
        """Return only published posts."""
        return self._posts

Or, I may be able to remove this and rename the related model to posts instead of _posts. I only called it _posts because I wanted to add some extra logic to the posts property. But this logic (only returning published posts) is now built into the default queryset of the manager.