Add data migration to purge deleted submissions

WARNING: may be slow on large databases. Mention this in release notes.
Closes #733.

onadata/apps/logger/migrations/0019_purge_deleted_instances.py

@@ -0,0 +1,42 @@
+# Generated by Django 2.2.14 on 2021-06-30 19:00
+from django.conf import settings
+from django.db import migrations
+
+
+def purge_deleted_instances(apps, schema_editor):
+    """
+    Remove all submissions that have been already marked as deleted from both
+    PostgreSQL and MongoDB. If this is too slow, revert to a previous release
+    and run the code in
+    https://github.com/kobotoolbox/kobocat/issues/696#issuecomment-809622367
+    using `manage.py shell_plus`.
+    """
+    Instance = apps.get_model('logger', 'Instance')
+    to_purge = Instance.objects.exclude(deleted_at=None).only('pk')
+    if not to_purge.exists():
+        return
+    print(
+        f'Purging {to_purge.count()} deleted instances...', end='', flush=True
+    )
+    for instance in to_purge.iterator():
+        # Manually delete from MongoDB because signals are not called in
+        # migrations (that would require freezing all application code, not
+        # just the models!)
+        settings.MONGO_DB.instances.delete_one({'_id': instance.pk})
+        instance.delete()
+    print('Done!', flush=True)
+
+
+def do_nothing(apps, schema_editor):
+    pass
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ('logger', '0018_add_submission_counter'),
+    ]
+    operations = [
+        migrations.RunPython(
+            purge_deleted_instances, reverse_code=do_nothing
+        ),
+    ]