Consider Primary Entity Reference for channels field

Problem/Motivation

See https://www.drupal.org/project/primary_entity_reference

This is a custom field type, which stores a flag for a primary item as an additional field property.

It would be more robust than our current system of taking the first value of the channels field to be the primary.

However, it has a few bugs I consider showstoppers, and it's only for D11.1+.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes