Customize: Reject a changeset update when a non-future date is provided and...

Customize: Reject a changeset update when a non-future date is provided and also ensure that a published changeset always gets set to the current date/time.

* Also moves checks from `customize_save` Ajax handler to the underlying `WP_Customize_Manager::save_changeset_post()` call which plugins may invoke directly.
* Ensures that `customize_save_response` filter is always passed an array, with error code available as `code`.

Props utkarshpatel, westonruter, sayedwp.
See #30937.
Fixes #38943.

Built from https://develop.svn.wordpress.org/trunk@39409


git-svn-id: https://core.svn.wordpress.org/trunk@39349 1a063a9b-81f0-0310-95a4-ce76da25c4cd