Customize: Allow 0:00-0:59 in date/time control when 24-hour time used.

* Let min hour be 0 and max be 23 in 24-hour time; let min hour be 1 and max be 12 in 12-hour time.
* Show error notification when an invalid date value is provided, not just when not a future date.
* Fix translation of custom validity message.
* Start checking for validity after all inputs have been initially populated.
* Remove support for being able to enter 24:00.
* Cease forcing date input elements from being casted to integers, to allow for invalid inputs to be detected.

Props westonruter, Presskopp, peterwilsoncc, atachibana for testing.
See #39896, #28721.
Fixes #42373.

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git-svn-id: https://core.svn.wordpress.org/trunk@41876 1a063a9b-81f0-0310-95a4-ce76da25c4cd