🚸(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/enums.py

@@ -6,6 +6,14 @@ from django.db import models
 from django.utils.translation import gettext_lazy as _
 
 
+class MailDomainRoleChoices(models.TextChoices):
+    """Defines the possible roles a user can have to access to a mail domain."""
+
+    VIEWER = "viewer", _("Viewer")
+    ADMIN = "administrator", _("Administrator")
+    OWNER = "owner", _("Owner")
+
+
 class MailDomainStatusChoices(models.TextChoices):
     """Defines the possible statuses in which a mail domain can be."""