Copy Post: add option to copy post/page to Post-publish panel

Right now, folks must go to the Posts list screen to be offered the option to copy a post or page. In some scenarios, one may want to copy a page right after publishing it. We could add a button to copy the post / page on the post publish panel.

Requested here:
https://wordpress.com/forums/topic/wordpress-publishing-suggestion-or-please-direct-me-where-i-can-make-one/?view=all#post-4053512

Start at the existing copy option on the Posts list screen and trace the post-publish panel entry point. Confirm how copying works for both posts and pages, then add the corresponding post-publish action and verify that it is available immediately after publishing.

Written by the indexing model from the issue text.