This stores the components as children of the item that contains them via the DB's `parent_container_item_id` feature, and ensures that things are loaded in a good order with breadth-first search.
Squahed fixes:
- Fix some constraint violations that occurred when swapping inventory items.
- Comment out recipes for modular weapons.
- Make update_item_at_slot_using_persistence_key and is_modular more idiomatic.
- Add changelog entry.
- Document `defer_foreign_keys` usage.