Use date time control from 4.9-alpha for post date

[westonruter]

We should open a PR to try integrating the date time control from 4.9-alpha to confirm that it works in that use case.

Also we should make sure that the date time control can be used statically with an arbitrary setting, like an "established date" without even a time.

Depends on https://core.trac.wordpress.org/ticket/37964 since the post_date will not be a separate setting registered in and of itself.