Add "Are you sure?" page for register absence form

I just had the case that a teacher marked a person as absent for a time of a month, because of a single mistake.

changed milestone to %1.0a1

added part::frontend prio::1 source::customer type::feature labels

Maybe write a Mixin for the form Class, that creates this page if a certain condition applies

Yes, this should be implemented globally and reusable. I am pretty sure there is prior art for that.

assigned to @hansegucker

created merge request !109 (merged) to address this issue

mentioned in merge request !109 (merged)

mentioned in commit 7b4cd390

closed with merge request !109 (merged)