fix(compat): Use ForeignObjectRel.hidden field from django 5.1

[last-partizan]

Refs #719

[Freakness109]

If you don't mind me adding the results of some investigations:
The main issue seems to be that accessor_name on ForeignObjectRel got turned into a cached property (django/django@5e80390, upstream https://code.djangoproject.com/ticket/35230). That means that as soon as related_name is overwritten in e.g. fields.py, the accessor_name will have an out-of-date string. Not sure how to fix it short of updating accessor_name manually.