Resolve "Filter selects on week overview and register absence"

Closes #79 (closed)

aleksis/apps/alsijil/util/alsijil_helpers.py

 from typing import Optional
 
+from calendarweek import CalendarWeek
 from django.http import HttpRequest
 
 from aleksis.apps.chronos.models import LessonPeriod
@@ -13,8 +14,9 @@ def get_lesson_period_by_pk(
     period_id: Optional[int] = None,
 ):
     """Get LessonPeriod object either by given object_id or by time and current person."""
+    wanted_week = CalendarWeek(year=year, week=week)
     if period_id:
-        lesson_period = LessonPeriod.objects.get(pk=period_id)
+        lesson_period = LessonPeriod.objects.annotate_week(wanted_week).get(pk=period_id)
     elif hasattr(request, "user") and hasattr(request.user, "person"):
         if request.user.person.lessons_as_teacher.exists():
             lesson_period = (