🚸(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/admin.py

@@ -25,7 +25,13 @@ class MailDomainAdmin(admin.ModelAdmin):
 class MailDomainAccessAdmin(admin.ModelAdmin):
     """Admin for mail domain accesses model."""
 
-    list_display = ("user", "domain")
+    list_display = (
+        "user",
+        "domain",
+        "role",
+        "created_at",
+        "updated_at",
+    )
 
 
 @admin.register(models.Mailbox)