Form fields for pre-approved submitters only

[goldpbear]

For the pboakland flavor, we have a use case for form fields that are visible only to logged-in, pre-approved submitters.

This functionality is very similar to what we want from #499, but that issue pertains to the editor/admin panel, which is not ready to be deployed to a live flavor. Instead, we need a simpler standalone feature suitable for deployment to pboakland.