migrations: Index MutedTopic on stream/topic.

This index lets us quickly find all users who may be muting
a topic, which useful when we send messages out.

zerver/migrations/0112_index_muted_topics.py

@@ -0,0 +1,21 @@
+# -*- coding: utf-8 -*-
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0111_botuserstatedata'),
+    ]
+
+    operations = [
+        migrations.RunSQL(
+            '''
+            CREATE INDEX zerver_mutedtopic_stream_topic
+            ON zerver_mutedtopic
+            (stream_id, upper(topic_name))
+            ''',
+            reverse_sql='DROP INDEX zerver_mutedtopic_stream_topic;'
+        ),
+    ]