refactor timeslot API filters

This MR:

• removes default values for start and end filter (see #189 (closed))
• renames start to startsAfter and end to endsBefore
• adds datetime support to startsAfter and endsBefore
• adds new datetime filter startsBefore and endsAfter

program/filters.py

@@ -184,6 +184,11 @@ class TimeSlotFilterSet(filters.FilterSet):
             "If specified without a datetime value the current date and time is assumed."
         ),
     )
+    starts_before = filters.DateTimeFilter(
+        field_name="start",
+        lookup_expr="lt",
+        help_text="Only returns timeslots that start before the specified datetime.",
+    )
     starts_after = filters.DateTimeFilter(
         field_name="start",
         lookup_expr="gte",
@@ -194,6 +199,11 @@ class TimeSlotFilterSet(filters.FilterSet):
         lookup_expr="lt",
         help_text="Only returns timeslots that end before the specified datetime.",
     )
+    ends_after = filters.DateTimeFilter(
+        field_name="end",
+        lookup_expr="gte",
+        help_text="Only returns timeslots that end at or after the specified datetime.",
+    )
     schedule_ids = IntegerInFilter(
         field_name="schedule",
         help_text="Return only timeslots that belong to the specified schedule(s).",