Kits part 2 - Introduce KitItem and ConcreteItem with Single Table Inheritance

GitHub Issue #5517 - rubyforgood/human-essentials

This is the next step on the Kit roadmap (#3652). Create two new classes, KitItem and ConcreteItem, and turn Item into a parent class by adding a type column and Single Table Inheritance (STI).

The association between kits and items should be renamed to kit_item to make the relationship more explicit.

The following PR will switch from creating/updating kits to creating/updating KitItems, and move the sync from kit -> item to kit_item -> kit for backwards compatibility.