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

@@ -541,7 +541,7 @@ class APITimeSlotViewSet(viewsets.ModelViewSet):
         # If someone tries to sort by a field that isn't available on the model
         # we silently ignore that and use the default sort order.
         model_fields = [field.name for field in TimeSlot._meta.get_fields()]
-        if order not in model_fields:
+        if order.lstrip('-') not in model_fields:
             order = default_order