Creating a new PublishableEntity should automatically create a row in Draft and Published

[ormsbee]

NOTE: This is not ready for work yet -- see the note at the bottom.

In a recent PR, there was a need for code like this:

current_draft: Optional[Draft] = getattr(entity, "draft", None)
current_published: Optional[Published] = getattr(entity, "published", None)

This is because creating a new PublishableEntity does not populate these other models:

https://github.com/openedx/openedx-learning/blob/ae597431892807b65243eba0b835e515c58da077/openedx_learning/apps/authoring/publishing/api.py#L183-L205

I think I originally wanted to distinguish between never published and published-but-since-unpublished. But now we have a whole PublishLog to track that.

Problem: The Published model actually has a non-nullable reference to a PublishLogRecord that points to how this particular version became the published version. If we initialize a Published entry to null, we would have to make this field nullable as well. Is that tradeoff worth it, or should we just make helpers on PublishableEntity to return the draft and published versions instead?