Verified_fields: Allow librarians to lock certain work/edition fields

### Describe the problem that you'd like solved

As of:
- #7495
...when we encounter new data (such as new publishers) the importer biases towards appending them to the list. In some cases where a librarian has verified a publisher (or book cover) we may want to lock these fields to prevent confusion: as described in:
- #7744

For now, let's start with `publishers` and `covers` as fields librarians can lock
- the UI might introduce a checkbox (librarian-only) on the /edit page next to covers and publishers to lock these fields
- the importer should ignore importing fields which existing the `verified_fields` array
- the `verified_fields` should exist on both the work and the edition

### Stakeholders

@seabelis

Start at the /edit page, the importer, and the work and edition data models mentioned in the issue. Trace how publishers and covers are edited and imported, then verify that librarian-only controls can mark both fields and that imports preserve fields listed in verified_fields.

Written by the indexing model from the issue text.