Merged in [16701] from rcross@amsl.com:

Add migration to remove any break / registration sessions that have already been created for future meetings.  Goes with changes for #2713.
 - Legacy-Id: 16816
Note: SVN reference [16701] has been migrated to Git commit 0bbdbca8ac

ietf/meeting/migrations/0019_remove_future_break_sessions.py

@@ -0,0 +1,33 @@
+# Copyright The IETF Trust 2019, All Rights Reserved
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.22 on 2019-07-22 14:56
+
+from __future__ import unicode_literals
+
+import datetime
+from django.db import migrations
+
+
+def forward(apps, schema_editor):
+    Meeting = apps.get_model('meeting', 'Meeting')
+    today = datetime.datetime.today()
+    meetings = Meeting.objects.filter(date__gt=today, type='ietf')
+    for meeting in meetings:
+        meeting.agenda.assignments.all().delete()
+        meeting.session_set.all().delete()
+        meeting.timeslot_set.all().delete()
+
+
+def backward(apps, schema_editor):
+    return
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('meeting', '0018_document_primary_key_cleanup'),
+    ]
+
+    operations = [
+        migrations.RunPython(forward, backward),
+    ]