Fix up several fields in the admin UI for models.

While adding some entries to the docs database for the diff file
attachments, I realized we had some missing fields in the admin UI and
some fields that should have been limited to raw IDs. The file
attachment fields on ReviewRequest weren't exposed at all.

This change adds the missing fields and marks up a few to require raw
IDs.

Also, some fields weren't allowed to be blank, even though they should
have been.