Following on from #413 and #416. Update the spares field in the parent catalogue item whenever:

• An item is created
• An item is deleted
• An item's usage status is updated
• The definition of a spare is updated

It is not currently possible to move items between catalogue items. The calculation itself should:

• Be done in a transaction - editing the spares field first to ensure write locked to prevent multiple conflicting updates (also need the inserts/update/delete to be done in the same transaction as well to ensure they cannot become out of sync)
• Consider https://stackoverflow.com/questions/60237515/mongodb-returning-wrong-count
• Raise an appropriate error like Raise a 409 error when a property migration fails due to a write conflict #412 if a conflict occurs (https://stackoverflow.com/questions/65886341/mongo-throwing-write-conflicts-for-concurrent-sessions - For now we wont retry the database as should be extremely rare and avoids a time.sleep)