Purchase Order Number and Item Descriptions on Draft Contracts

Draft contracts now carry the same purchase order and description fields as contracts.

• POST /draft-contracts and PUT /draft-contracts/{draft_contract_id} accept an optional
  purchase_order_number (up to 50 characters) and an optional description on each item, which
  overrides the product description on invoice lines once the draft becomes a contract.
• Draft contract responses return both fields. purchase_order_number is null when none was set and an
  item's description is null when the product description applies.

Applies to V4 only.