models: Add unique constraint on RealmUserDefault.realm.

This model is by designed intended to exist on a 1:1 relationship with
Realms, and we attempt to ensure that with application code, but we
should have a unique constraint too, since a database with duplicate
such entries would be corrupted.

We do this via the standard Django OneToOneField.

zerver/migrations/0378_alter_realmuserdefault_realm.py

@@ -0,0 +1,21 @@
+# Generated by Django 3.2.12 on 2022-03-05 02:59
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("zerver", "0377_message_edit_history_format"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="realmuserdefault",
+            name="realm",
+            field=models.OneToOneField(
+                on_delete=django.db.models.deletion.CASCADE, to="zerver.realm"
+            ),
+        ),
+    ]