Updated rollback functionality to not remote embargo/expiry

[chrispenny]

After some discussion it was decided that we should not dictate that an Embargo/Expiry date on old versions should be removed on rollback. We have instead updated the status message to be a "warning", with appropriate messages:

This message will show up in the History tab as authors browse through versions.

Importantly, this behaviour also matches the behaviour found in the SilverStripe 4 version of the Advanced Workflow module.

Developers should be free to add their own rollback behaviour to determine whether or not these dates are removed. Though, I do have an open issue on Versioned questioning if there is an appropriate place to implement this sort of behaviour: https://github.com/silverstripe/silverstripe-versioned/issues/170

[chrispenny]

@chillu this might be of interest to you, as you originally requested removal of Embargo/Expiry dates on rollback.

TL;DR:

• We no longer dictate this behaviour, but we offer warnings to authors.
• Not removing E&E on rollback matches the functionality of the SS4 version of Advanced Workflow (tested on hackday).