Fixed #23431 -- Allowed inline and hidden references to admin fields.

This fixes a regression introduced by the 53ff096982 security fix.

Thanks to @a1tus for the report.

refs <a target="_new" href="https://code.djangoproject.com/ticket/23329">#23329</a>.
maybe you'd like to include the ticket number here so there' s a link between "Models for #" and "Test for #"
Otherwise, LGTM minus the release notes. I guess we should do one more 1.5.x. release...