[Kit Redesign] Phase 1 - Change itemizable source

[dorner]

Summary

See https://github.com/rubyforgood/human-essentials/issues/3652

• Add the Itemizable module to the base Item class.
• Add a migration to change the itemizable_id for all kits' line items to instead point to the kit's item. Back up the DB first!
• Update views and allocation / deallocation logic to work with the item's line items instead of the kits.

Things to consider

We should probably add a comment to the Itemizable module stressing that it indicates that the parent model has line items, not items.

Criteria for Completion

No response