Include author on product prices

Include an author node on fixed and unit prices, to track who created them.
This will store the user id in a details node, as an interim solution until create/update users are stored in actual database columns. It is therefore not ideal for reporting on but:

• can be used to display the user name in the UI

• can be migrated when the database model changes

It will need to be populated when editing and importing prices, and when new prices are created by delivery processing