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/min_max.py

-from django.db.models import Min, Max
-
-from aleksis.apps.chronos.models import TimePeriod
-
-# Determine overall first and last day and period
-min_max = TimePeriod.objects.aggregate(
-    Min("period"), Max("period"), Min("weekday"), Max("weekday"), Min("time_start"), Max("time_end")
-)
-
-period_min = min_max.get("period__min", 1)
-period_max = min_max.get("period__max", 7)
-
-time_min = min_max.get("time_start__min", None)
-time_max = min_max.get("time_end__max", None)
-
-weekday_min_ = min_max.get("weekday__min", 0)
-weekday_max = min_max.get("weekday__max", 6)
-
-