Add UninstallRenitentPolymorphicManager

This manager uses django-polymorphic's `instance_of` QuerySet method
to filter for models that are known subclasses at the time of execution.

aleksis/core/managers.py

@@ -5,6 +5,7 @@ from django.contrib.sites.managers import CurrentSiteManager as _CurrentSiteMana
 from django.db.models import QuerySet
 
 from calendarweek import CalendarWeek
+from polymorphic.managers import PolymorphicManager
 
 
 class CurrentSiteManagerWithoutMigrations(_CurrentSiteManager):
@@ -91,3 +92,10 @@ class GroupManager(CurrentSiteManagerWithoutMigrations):
 
 class GroupQuerySet(SchoolTermRelatedQuerySet):
     pass
+
+
+class UninstallRenitentPolymorphicManager(PolymorphicManager):
+    """A custom manager for django-polymorphic that filters out submodels of unavailable apps."""
+
+    def get_queryset(self):
+        return super().get_queryset().instance_of(*self.model.__subclasses__())

aleksis/core/models.py

@@ -35,6 +35,7 @@ from .managers import (
     GroupManager,
     GroupQuerySet,
     SchoolTermQuerySet,
+    UninstallRenitentPolymorphicManager,
 )
 from .mixins import (
     ExtensibleModel,
@@ -737,6 +738,8 @@ class DashboardWidget(PolymorphicModel, PureDjangoModel):
           )
     """
 
+    objects = UninstallRenitentPolymorphicManager()
+
     data_checks = [BrokenDashboardWidgetDataCheck]
 
     @staticmethod