Calculate the number of spares within a catalogue item

[joelvdavies]

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

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
• [ ] Consider https://stackoverflow.com/questions/60237515/mongodb-returning-wrong-count
• [ ] Raise an appropriate error like #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)