👔(dimail) allow creation of "pending" mailboxes

Previously, mailbox creation was restricted to "enabled" domains.
We now allow users to create mailboxes on pending and failed domains.
Mailboxes thus created have the "pending" mailboxes status.

src/backend/mailbox_manager/enums.py

@@ -1,3 +1,4 @@
+# pylint: disable=too-many-ancestors
 """
 Application enums declaration
 """
@@ -21,3 +22,12 @@ class MailDomainStatusChoices(models.TextChoices):
     ENABLED = "enabled", _("Enabled")
     FAILED = "failed", _("Failed")
     DISABLED = "disabled", _("Disabled")
+
+
+class MailboxStatusChoices(models.TextChoices):
+    """Lists the possible statuses in which a mailbox can be."""
+
+    PENDING = "pending", _("Pending")
+    ENABLED = "enabled", _("Enabled")
+    FAILED = "failed", _("Failed")
+    DISABLED = "disabled", _("Disabled")