Let experiment leaders create (empty) time slots

[bbonf]

For scheduling appointments there should be two supported flows:

1. Experiment leader calls parent and immediately makes an appointment

   • This results in both a TimeSlot and Appointment objects being created (and linked)

2. Experiment leader makes an empty time slot, parent later potentially schedules an appointment in an empty time slot

   • In this case there should first be just a TimeSlot object, without an appointment
   • It should appear in the agenda view(s) in the lab application
   • It should be possible in a new view for experiment leaders ("Create time slots" or something like that)
   • (In a separate issue: it should appear in an agenda view in the parent application, where the parent will schedule an appointment)

Flow 1 already exists in the app