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>.
I like to put the ticket inline with the description: `#23431 - Specifying...`.