[manual] Add script to migrate subs to new stream notification settings.

Previously, streams used `notifications`. Now they have separate
`desktop_notifications` and `audible_notifications`.

On staging, this should be run after the schema migration has been
applied (and technically before the code that uses the new fields is
deployed, but for staging, processing our notification settings a few
minutes late won't hurt anyone).

On prod, the script should be re-run just before the code using the
new notification settings is deployed to process any customer
notification settings changed since the staging deploy.

(imported from commit d99d238cd1b317c5180d7f940d70a7e2f8f9c712)

zilencer/management/commands/migrate_stream_notifications.py

@@ -0,0 +1,14 @@
+from __future__ import absolute_import
+
+from django.core.management.base import BaseCommand
+from zerver.models import Subscription
+
+class Command(BaseCommand):
+    help = """One-off script to migration users' stream notification settings."""
+
+    def handle(self, *args, **options):
+        for subscription in Subscription.objects.all():
+            subscription.desktop_notifications = subscription.notifications
+            subscription.audible_notifications = subscription.notifications
+            subscription.save(update_fields=["desktop_notifications",
+                                             "audible_notifications"])