Create new user or guest author while assigning to a post

[danielbachhuber]

Originally from the forums, it would be neat if you could create a user or guest author on the fly while assigning to a post. For example, if you were to do a search for a user that doesn't exist, there would be a small prompt to create a new user/ guest author in a modal or the like.

[emgk]

Hello @philipjohn and @danielbachhuber as per the enhancement request I added the code to create a new guest author if doesn't exist while assigning to the post on post edit page.

Please check the PR above and do let me know if any changes needed! :)