Fix: restrict order detail price updates to current order

[stonebuzz]

Checklist before requesting a review

Please delete options that are not relevant.

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• When saving the detail lines of an order, the submitted price and discount values were applied to any item id received, even when that item belonged to another order.
  A user editing one order could therefore alter the prices of items attached to an order they are not allowed to access.
  Each submitted item is now checked against the order being edited, and lines that do not belong to it are ignored.

Screenshots (if appropriate):