Cannot clear a scheduled publication date

[allanlasser]

As @eyeseast discovered during his demo, scheduled status changes are like the Hotel California: you can schedule a change, but you can never unschedule it.

We can add a button alongside the field to clear it when a value is set. We also need to make sure the field is not marked "required" so it can be submitted while empty.

[eyeseast]

While we're looking at this, we should also add an indicator near the access icon that says if a scheduled publish date exists.