[IMP] purchase(_stock): Add more and update PO dates

Adds "Expected Date" and "Effective Date" fields to purchase orders to
match what exists in sales orders. "Effective Date" mirrors sales logic
(= first completed stock move). "Expected Date" has simplier logic due
to simplier nature of receiving stock vs shipping (= earliest "Scheduled
Date" of all PO lines). Form view has been updated to include these new fields.

Additionally code was changed to not have PO 'order_line.date_planned' be
overwritten by the 'purchase_order.date_planned' value. Previous behavior
was both confusing and made it so info was lost. Also logic for related
stock move picking line expected date values has been updated to now consider
'purchase_order.date_planned' value if set. Related changes include:
    - not overwriting when PO is edited+saved (this only occurred in write()
      logic, not create() which caused inconsistent behavior anyways.),
    - updating related tests,
    - removing PO line date_planned editing restriction when PO date_planned
      is set
    - updating of relevant stock moves/picking expected date due to
      PO.line.propogate_date = True ('purchase_order.date_planned' if
      exists, otherwise 'order_line.date_planned')

This change also includes some small related improvements:
- Removal of outdated code comment.
- Addition of Help strings to better explain fields.
- Prevent copy of 'date_planned' value into new POs (now mirrors sales
  logic).

This change corresponds to "Receipt Date on PO's" subsection of Purchase
KPIs task.

Task #2198420

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