Separate publication status from workflow status

[greglinch]

Problem

If an updated post needs to be edited after publication, having custom statuses tied to publication status doesn't let the writer revert the workflow status to pending review without the post unpublishing.

Proposed solution

Separate the workflow statuses from the publication status. For example, "pending review" or other custom statuses would have no affect on whether the post is published or not.

[AramZS]

@greglinch For related reading, fun convo on this in core.trac - https://core.trac.wordpress.org/ticket/12706#comment:183

[greglinch]

@AramZS Thanks for sharing that!

[philipjohn]

There will be cases where any changes to already published content will need to go through the editorial workflow again, so this would certainly be a good improvement.

[htdat]

The idea of cloning/forking posts can resolve the problem here #400