WIP Deduploads

Work in progress to deduplicate loads of adjustments via enforcing the same object graph walking on each adjustment access.
I think a nicer way to write this would be:

```ruby
def best_promotion_adjustment
  @best_promotion_adjustment ||= adjustments.promotion.eligible.sort_by do |promotion|
    [promotion.amount, Time.at(0) - promotion.created_at]
  end.first
end
```

You could even define a constant for `Time.at(0)` to avoid recalculating it each time.