API Ability to reschedule existing embargo / unpublished dates

Responding to a user complaint, it seems obvious to me that a scheduled future publish date should be able to be changed. I'm unsure as to why it was necessary for this to cause a validation error in the first place. Having to wait until the embargo has been reached before changing it seems unreasonable.

Unit tests have been added, and I have run through a manual test to confirm that there were no obvious usability issues.

@phptek you wrote the original validation code at https://github.com/silverstripe-australia/advancedworkflow/pull/68. Are you able to provide a bit more justification as to why it was necessary?