Add ability to get all of an order's adjustments easily
[backend] Improve adjustments interface

- Shows items that are adjusted and precisely how they are adjusted
- Removes date/time, since this is not really that relevant for automatic adjustments
Add short comment explaining what immutable means in Adjustment model
Ensure tax category matches when creating a new tax_adjustment from the factory

This ensures that the adjustment amount is not $0, but rather the correct amount.