confirmation: Add database indexes on confirmation fields.

Apparently, we didn't have any database indexes on Confirmation, which
meant that on servers with large numbers of users like zulipchat.com,
new account registration could spend a ton of time effectively doing a
table scan on this table.

confirmation/migrations/0007_add_indexes.py

@@ -0,0 +1,37 @@
+# Generated by Django 2.2.10 on 2020-03-27 09:02
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('confirmation', '0006_realmcreationkey_presume_email_valid'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='confirmation',
+            name='confirmation_key',
+            field=models.CharField(db_index=True, max_length=40),
+        ),
+        migrations.AlterField(
+            model_name='confirmation',
+            name='date_sent',
+            field=models.DateTimeField(db_index=True),
+        ),
+        migrations.AlterField(
+            model_name='confirmation',
+            name='object_id',
+            field=models.PositiveIntegerField(db_index=True),
+        ),
+        migrations.AlterField(
+            model_name='realmcreationkey',
+            name='creation_key',
+            field=models.CharField(db_index=True, max_length=40, verbose_name='activation key'),
+        ),
+        migrations.AlterUniqueTogether(
+            name='confirmation',
+            unique_together={('type', 'confirmation_key')},
+        ),
+    ]