verbose_field_name broken for reverse rels w/o related_name

[rpkilby]

I'm surprised I haven't noticed this yet, but verbose_field_name is broken for reverse relationships that don't set related_name. This is an extension of #716. Example:

class Article(models.Model):
    author = models.ForeignKey(User, on_delete=models.CASCADE)
    ...

verbose_field_name(User, 'article') should return article, but chokes.

I need to look into this further, but I'm thinking the following:

django-filter/django_filters/utils.py

Lines 250 to 256 in 7c08dee

	if isinstance(part, ForeignObjectRel):
	if part.related_name:
	names.append(part.related_name.replace('_', ' '))
	else:
	return '[invalid name]'
	else:
	names.append(force_text(part.verbose_name))

should be:

    if isinstance(part, ForeignObjectRel): 
        meta = part.related_model._meta
        names.append(force_text(meta.verbose_name))
    else:
        names.append(force_text(part.verbose_name))

The downside would be that you'd lose the context of the reverse related name. Need to think about it, but not sure if it'd be more or less correct. Alternatively, we could do something like:

    if isinstance(part, ForeignObjectRel): 
        if part.related_name: 
            # not sure if pretty_name or just .replace(...)
            names.append(pretty_name(meta.verbose_name))
        else:
            meta = part.related_model._meta
            names.append(force_text(meta.verbose_name))
    else:
        names.append(force_text(part.verbose_name))