Fix web oncall calendars not loading (#891)

* Fix oncall schedules bug

* Update custom_on_call_shift.py

engine/apps/schedules/models/custom_on_call_shift.py

@@ -373,6 +373,8 @@ def convert_to_ical(self, time_zone="UTC", allow_empty_users=False):
                     expected_start_day = min(CustomOnCallShift.ICAL_WEEKDAY_REVERSE_MAP[d] for d in self.by_day)
                     delta = (expected_start_day - start.weekday()) % 7
                     start = start + timezone.timedelta(days=delta)
+                    if self.until is not None:
+                        self.until = self.until + timezone.timedelta(days=delta)
 
             if self.frequency == CustomOnCallShift.FREQUENCY_DAILY and self.by_day:
                 result = self._daily_by_day_to_ical(time_zone, start, users_queue)