Rework UI per private discussion

Problem/Motivation

It does seem like a pretty odd editorial experience that a user can see a referenced item, remove it, and then not be able to add it back again. A better UX may be to provide a value type that contains unpublished media items, so the unprivileged user would never even see them to begin with, then silently merge data in the back-end.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes