Unify API of lesson period with event and extra lesson by adding properties for group names

To be able to use the same API on the models lesson period, event and extra lesson which are often used in a common context (e. g. in Alsijil), I added the properties group_names and group_short_names to the lesson period model. They will use the connected lesson object for getting the real data.

aleksis/apps/chronos/models.py

@@ -473,6 +473,16 @@ class LessonPeriod(WeekAnnotationMixin, TeacherPropertiesMixin, ExtensibleModel)
     def get_groups(self) -> models.query.QuerySet:
         return self.lesson.groups
 
+    @property
+    def group_names(self):
+        """Get group names as joined string."""
+        return self.lesson.group_names
+
+    @property
+    def group_short_names(self):
+        """Get group short names as joined string."""
+        return self.lesson.group_short_names
+
     def __str__(self) -> str:
         return f"{self.period}, {self.lesson}"