confirmation: Migration to add expiry_date step 1.

The commit:
1. Adds the new field as nullable.
2. Adds code that'll create new Confirmation with the field set
   correctly.
3. For verifying validity of Confirmation object this still uses the old
   logic in get_object_from_key() to keep things functioning until we
   backfill the old objects in the next step.

Thus this commit is deployable. Next we'll have a commit to run a
backfill migration.

confirmation/migrations/0008_confirmation_expiry_date.py

@@ -0,0 +1,17 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("confirmation", "0007_add_indexes"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="confirmation",
+            name="expiry_date",
+            field=models.DateTimeField(db_index=True, null=True),
+            preserve_default=False,
+        ),
+    ]