migrations: Fix 0301 to replace a Python loop with SQL.

The previous code is correctly flagged by semgrep 0.23 as a violation
of our sql-format rule.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/migrations/0301_fix_unread_messages_in_deactivated_streams.py

@@ -1,21 +1,6 @@
-from django.db import connection, migrations
-from django.db.backends.postgresql.schema import DatabaseSchemaEditor
-from django.db.migrations.state import StateApps
+from django.db import migrations
 
 
-def mark_messages_read(apps: StateApps, schema_editor: DatabaseSchemaEditor) -> None:
-    Stream = apps.get_model("zerver", "Stream")
-    deactivated_stream_ids = list(Stream.objects.filter(deactivated=True).values_list('id', flat=True))
-    with connection.cursor() as cursor:
-        for i in deactivated_stream_ids:
-            cursor.execute(f"""
-                UPDATE zerver_usermessage SET flags = flags | 1
-                FROM zerver_message
-                INNER JOIN zerver_stream ON zerver_stream.recipient_id = zerver_message.recipient_id
-                WHERE zerver_message.id = zerver_usermessage.message_id
-                AND zerver_stream.id = {i};
-            """)
-
 class Migration(migrations.Migration):
     """
     We're changing the stream deactivation process to make it mark all messages
@@ -23,12 +8,19 @@ class Migration(migrations.Migration):
     deactivated before this change, we need a migration to fix those old streams,
     to have all messages marked as read.
     """
-    atomic = False
-
     dependencies = [
         ('zerver', '0300_add_attachment_is_web_public'),
     ]
 
     operations = [
-        migrations.RunPython(mark_messages_read, reverse_code=migrations.RunPython.noop),
+        migrations.RunSQL(
+            sql="""
+                UPDATE zerver_usermessage SET flags = flags | 1
+                FROM zerver_message
+                INNER JOIN zerver_stream ON zerver_stream.recipient_id = zerver_message.recipient_id
+                WHERE zerver_message.id = zerver_usermessage.message_id
+                AND zerver_stream.deactivated;
+            """,
+            reverse_sql="",
+        ),
     ]