Customize: Always use underlying changeset post date as initial date in...

Customize: Always use underlying changeset post date as initial date in scheduled date fields if date is future.

Prevent user from having to re-enter the previously-supplied future date if they had switched from schedule to draft but wanted to switch back again. Improves parity with behavior of post edit screen.

Props dlh, westonruter, obenland.
See #39896, #28721.
Fixes #42413.

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git-svn-id: https://core.svn.wordpress.org/trunk@41938 1a063a9b-81f0-0310-95a4-ce76da25c4cd