[Kit Redesign] Phase 1 - Change itemizable source

[dorner]

Summary

See #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

[zeeshan-haidar]

I can work on this

[github-actions]

This issue is marked as stale due to no activity within 30 days. If no further activity is detected within 7 days, it will be unassigned.

[zeeshan-haidar]

This issue depends on this PR to get merge first

[jimmyli97]

I think I can do this by cherry-picking commits from #3750 and fixing it up a little

[awwaiid]

Update -- we got some of the smaller refactors in previously, but the larger refactor remains if someone wants to try again. We've made changes to how we record events and base items, so if you do tackle this (hello future volunteer!) then please write out a short plan or identify some small changes that can be merged independently toward this end.

[dorner]

Fixed with #5426.