Replace RelationDiffer with RelationList callbacks

[unclecheese]

The RelationDiffer class is a really heavy-handed solution for ensuring snapshots are created when a many many changes on a page (e.g. adding / removing categories in a checkboxsetfield). Additionally, this class is used to navigate the anomaly of page saves resulting in elemental editor saves.

Now that we have callbacks for add/remove on relation lists, maybe this is a more elegant solution and we can deprecate RelationDiffer. I have a POC working in the silverstripe-gatsby module.