[1.8.x] Fixed #24381 -- removed ForeignObjectRel opts and to_opts

These cached properies were causing problems with pickling, and in
addition they were confusingly defined: field.rel.model._meta was
not the same as field.rel.opts.

Instead users should use field.rel.related_model._meta inplace of
field.rel.opts, and field.rel.to._meta in place of field.rel.to_opts.

Backport of f95122e541 from master

django/contrib/admin/utils.py

@@ -316,7 +316,7 @@ def label_for_field(name, model, model_admin=None, return_attr=False):
             label = field.verbose_name
         except AttributeError:
             # field is likely a ForeignObjectRel
-            label = field.opts.verbose_name
+            label = field.related_model._meta.verbose_name
     except FieldDoesNotExist:
         if name == "__unicode__":
             label = force_text(model._meta.verbose_name)