Make scheduling preparation easier

• This implements #144 (closed)
  • Don't show slots, slot creation button, and availabilities for wishes on detail page
  • Don't create slots for wishes automatically (and don't ask for durations)
  • Automatically create one slot when wish changes to AK
  • Automatically remove all unscheduled slots when AK changes to wish
• Introduce new generic admin actions with intermediate pages (preview/confirmation)
• Implements #145 (closed)
  • Admin interaction to delete slots for wishes
  • Admin interaction to create default availabilities
• Speedup and improve view for attention requiring AKs