common and timeslots apps now accept one constraint per definition, notice or booking

Signed-off-by: Marcel Oomens <Marcel.Oomens@cromido.org>
Bug fixes and improvements

- Fixed bug in timeslots/utils.py that caused HTTP/500 when no definitions are available
- Refactored some of timeslotPickerController.js

Signed-off-by: Marcel Oomens <Marcel.Oomens@cromido.org>