Invitations: fix model name (object_type) (#9837)

ContentType.name is the human-readable name,
this is taken from the label, which in some models
is a translatable string (https://github.com/readthedocs/readthedocs.org/blob/a00eff14643eeb1f114f6bcc6d71510e5c3d51fd/readthedocs/projects/models.py#L468-L468).

What we actually want is ContentType.model
https://docs.djangoproject.com/en/4.1/ref/contrib/contenttypes/#django.contrib.contenttypes.models.ContentType,
that is the plain model name in lowercase.

Author: stsewd

readthedocs/invitations/models.py

@@ -207,7 +207,7 @@ def can_revoke_invitation(self, user):
 
     @property
     def object_type(self):
-        return self.content_type.name
+        return self.content_type.model
 
     @property
     def object_name(self):