[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 f95122e5 from master