Add case-insensitive index on UserProfile.email.

This fixes a performance issue looking up UserProfile objects for
realms with a large number of users in the case that a UserProfile
object is not in the cache.

Thanks to @dbiollo for the suggestion!

zerver/migrations/0006_zerver_userprofile_email_upper_idx.py

@@ -0,0 +1,16 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import models, migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0005_auto_20150920_1340'),
+    ]
+
+    operations = [
+        migrations.RunSQL("CREATE INDEX upper_userprofile_email_idx ON zerver_userprofile ((upper(email)));",
+                          reverse_sql="DROP INDEX upper_userprofile_email_idx;"),
+    ]