confirmation: Use BigInteger to match other models' "id" field.

Without this, Django builds queries with which compare tables' "id"
fields to `("confirmation_confirmation"."object_id")::bigint`; the
explicit cast prevents the index added in the previous commit from
being used.

confirmation/migrations/0015_alter_confirmation_object_id.py

@@ -0,0 +1,15 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("confirmation", "0014_confirmation_confirmatio_content_80155a_idx"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="confirmation",
+            name="object_id",
+            field=models.PositiveBigIntegerField(db_index=True),
+        ),
+    ]