Move min_max helpers to TimePeriod as properties

Closes #59 (closed) and #43 (closed) (by fixing the aggregate defaults while at it).

aleksis/apps/chronos/util/prev_next.py

@@ -5,7 +5,7 @@ from calendarweek import CalendarWeek
 from django.urls import reverse
 from django.utils import timezone
 
-from .models import TimePeriod
+from ..models import TimePeriod
 
 
 def get_next_relevant_day(day: Optional[date] = None, time: Optional[time] = None, prev: bool = False) -> date: