✨(mailboxes) manage bad secret sent to dimail API

- manage 403 returned by dimail API when mail domain secret is not valid
- improve some tests
- improve MailboxFactory to mock success for dimail API POST call
- override 403.html to return a nice failing error in django admin
- an error message is displayed on mailbox creation form of frontend

src/backend/mailbox_manager/utils/dimail.py

@@ -84,5 +84,12 @@ class DimailAPIClient:
                 mailbox.domain.name,
                 extra=extra,
             )
-
+        elif response.status_code == status.HTTP_403_FORBIDDEN:
+            logger.error(
+                "[DIMAIL] 403 Forbidden: please check the mail domain secret of %s",
+                mailbox.domain.name,
+            )
+            raise exceptions.PermissionDenied(
+                f"Please check secret of the mail domain {mailbox.domain.name}"
+            )
         return response