* add code to clean event's agendas lease/bookings
* add annotation helper method annotate_queryset_for_lock_code() to
compute corrects places statistics given a lock_code (excluding
bookings linked to this lock_code)
* use annotate_queryset_for_lock_code() in Datetimes and
MultipleAgendasDatetimes
* make event's fillslot method completely atomic and add mechanic for
handling the lock code
* removed handling of IntegrityError which cannot happen for events
* lock_code is for now not supported with RecurringFillslots
Benjamin Dauvergne
Valentin Deniaud