🚸(models) use a viewer role for MailDomainAccess

Rename member role to viewer role for MailDomainAccess.
A viewer is only allowed to see list of mailboxes
created for a domain. It makes more sense to name it viewer.

src/backend/mailbox_manager/migrations/0007_alter_maildomainaccess_role.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.0.6 on 2024-08-02 15:11
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('mailbox_manager', '0006_maildomain_status'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='maildomainaccess',
+            name='role',
+            field=models.CharField(choices=[('viewer', 'Viewer'), ('administrator', 'Administrator'), ('owner', 'Owner')], default='viewer', max_length=20),
+        ),
+    ]