Provide setting for leaving default blank on Datetime Timestamp widget when creating new entities

Based on previous work submitted here, I made some adjustments for 9.5.

I think this is actually a duplicate of #2940758 which has a few patches attached and really fixes the issue for fields.
Even if that issue is not 100% valid for the "Authored on" field - haven't checked - we might be better working on a patch requiring the one of #2940758 to first apply; otherwise I am a afraid we are duplicating work here.