cubicweb-conference #1481805 missing constraint: talk ends after it starts [validation pending]

without this constraint, you can put an end date < begin date and the talk does not show up in the schedule/calendar without you understanding why.

prioritynormal
typebug
done in0.10.2
load0.500
load left0.000
closed by#120c3897754e [schema] conference and talks have to start before they end (closes #1481805)