State: Set new post status to draft in initialization

[aduth]

Related: #4956 (review)

This pull request seeks to simplify the behavior for setting a new post's status to draft, taking advantage of existing mechanisms put in place for setting the "Auto Draft" title to an empty string. This has the advantage of removing several ad-hoc checks, and avoids both the dispatch itself, but more importantly the creation of an undo level during the first save (since the EDIT_POST action is considered a change to undo).

We can still rely on the behavior of isEditedPostNew to check the currentPost.status, which will reflect the saved value prior to the initialization action setting edits values. This has been reflected in updated unit tests simulating a real-world scenario.

Testing instructions:

Verify that there are no regressions in the behavior of saving a new post. Notably:

• Saving a new post should save it as draft status (can verify in the posts list or in the network response)
• Elements not shown for new posts are still not shown (e.g. permalink)

[youknowriad]

This makes a lot of sense. Thanks 👍