Add customizable ordering and new request type for timeslot queryset

Hello!

In this MR I added a way to customize the ordering of the TimeSlot queryset with a new order GET parameter. It defaults to sorting by reverse start date. (aka -start).

I also added a new query parameter surrounding which returns the 10 timeslots closest to the current date.

Since it's been a while since I've written Python, I'm not sure if this is the most safe/optimal way to do it, but I hope I did alright :D

If I should change anything, please let me know!

program/views.py

@@ -509,6 +509,7 @@ class APITimeSlotViewSet(viewsets.ModelViewSet):
     /timeslots/{pk} eturns the given timeslot (GET)
     /timeslots/?start={start_date}&end={end_date} returns timeslots within the time range (GET)
     /shows/{show_pk}/timeslots returns timeslots of the show (GET, POST)
+    /shows/{show_pk}/timeslots?surrounding returns the 10 nearest timeslots for the current date (GET)
     /shows/{show_pk}/timeslots/{pk} returns a timeslots by its ID (GET, PUT, DELETE)
     /shows/{show_pk}/timeslots/?start={start_date}&end={end_date} returns timeslots of the show within the time range
     /shows/{show_pk}/schedules/{schedule_pk}/timeslots returns all timeslots of the schedule (GET, POST)