[meta] Radioactivity CRUD operations

By which I mean, lifecycle management of Radioactivity entities that are referred to by Radioactivity Reference fields.

There are a number of issues caused by the current way of doing things, that all stand to be resolved if the Radioactivity Entities are managed differently.

This meta is to provide a parent that can be used to collect and track all those issues.

Please don't post patches here - just links to related issues.