Purchase Orders: Notes Move To The Shared Internal Notes System

Purchase-order notes now live in the same Internal Notes system every other object uses, so the notes card on the PO Details tab behaves the same way it does everywhere else in the app. Milestone progress updates stay on the milestone card and are separate from notes — the two are no longer tangled together.

Existing PO notes were migrated over with their original ids, timestamps, archived state, and author preserved.