Constrain valid from enrollment attribute input

Add functionality so that the administrator configuring an enrollment flow can force a valid-from attribute being collected by an enrollment form to be a date that is not in the past.

This enhancement could be done in multiple steps. The first step might be to enable the administrator to force the valid-from date to be any time that is not in the past, i.e. "now or future". A later step could enable the administrator to have more fine-grained control like "1 week from the time the petition is created". Eventually the functionality could support fine-grained control over both past, present, and future.