✨(mailbox) remove secondary email as required field

The secondary email address is no longer required for all creation
processes and we should not force the user to provide and store an
insecure email address.

src/backend/mailbox_manager/migrations/0025_alter_mailbox_secondary_email.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.1.8 on 2025-04-08 20:07
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('mailbox_manager', '0024_domaininvitation'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='mailbox',
+            name='secondary_email',
+            field=models.EmailField(blank=True, max_length=254, null=True, verbose_name='secondary email address'),
+        ),
+    ]