Capitalize option group names in LinkField choices
Revert "form rendering quickfix"

This reverts commit f2d6d6b39308f2ab5058d329f4edc4536aaf3b5f.

I don't think this does anything, and it seems to work without it.
no need to fetch the user when just checking for existence
Fix inheritance for LinkFields

Don't copy the link_registry when copying a LinkField. When inheriting
from a model with a LinkField, each field from the parent is deepcopied
and attached to the child. This means that the child gets a different
instance of its link_registry, and could not learn about models
registered later.