Resolve "Add export functionality to course book"

Closes #258 (closed)

aleksis/apps/alsijil/models.py

 from datetime import datetime
-from typing import Optional
+from typing import Optional, List
 
 from django.contrib.auth.models import User
 from django.core.exceptions import PermissionDenied
@@ -118,6 +118,9 @@ class Documentation(CalendarEvent):
         if self.course:
             return self.course.groups.all()
 
+    def get_teachers_short_names(self) -> List[str]:
+        return [teacher.short_name or teacher.name for teacher in self.teachers]
+
     def __str__(self) -> str:
         start_datetime = CalendarEvent.value_start_datetime(self)
         end_datetime = CalendarEvent.value_end_datetime(self)