add 'housekeeping' management command

src/core/management/commands/housekeeping.py

+from django.core.management.base import BaseCommand
+from django.utils import timezone
+
+from ...models.messages import DirectMessage
+
+
+class Command(BaseCommand):
+    def handle(self, *args, **options):
+        # clear all direct messages which are after their expiry date
+        deleted_msgs_count, _ = DirectMessage.objects.filter(autodelete_after__isnull=False, autodelete_after__lte=timezone.now()).delete()
+        print(f'Deleted {deleted_msgs_count} messages.')